| 
           Changes in related party loans (Detail) (USD $) 
        In Thousands, unless otherwise specified  | 
        12 Months Ended | |
|---|---|---|
| 
           Dec. 31, 2012 
         | 
        
           Dec. 31, 2011 
         | 
      |
| Statement [Line Items] | ||
| Balance, beginning of year | $ 9,047 | $ 6,825 | 
| Advances | 7,630 | 7,926 | 
| Repayments | (8,096) | (4,204) | 
| Participations | 3,819 | (1,500) | 
| Balance, end of year | $ 12,400 | $ 9,047 | 
| X | ||||||||||
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- Definition 
          Amount of participation sold to related party during the reporting period. No definition available. 
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| X | ||||||||||
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- Definition 
          The net cash inflow or outflow from the proceeds and repayments made on the long-term borrowing from related party where one party can exercise control or significant influence over another party; including affiliates, owners or officers and their immediate families, pension trusts, and such forth. Alternate caption: Proceeds from (Payments for) Advances from Affiliates. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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| X | ||||||||||
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- Definition 
          Receivables to be collected from (obligations owed to) related parties, net as of the balance sheet date where one party can exercise control or significant influence over another party; including affiliates, owners or officers and their immediate families, pension trusts, and so forth. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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| X | ||||||||||
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- Details 
          
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| X | ||||||||||
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- Definition 
          The cash outflow for the payment of a long-term borrowing made from a related party where one party can exercise control or significant influence over another party; including affiliates, owners or officers and their immediate families, pension trusts, and so forth. Alternate caption: Payments for Advances from Affiliates. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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