Summary of Derivative Financial Instruments and Balances (Detail) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Derivatives, Fair Value [Line Items]
Notional Amount	$ 703,286	$ 578,959
Estimate Fair Value Gain	16,787	14,974
Estimate Fair Value Loss	(19,658)	(17,935)
Fair Value Hedges
Derivatives, Fair Value [Line Items]
Notional Amount	703,286	578,959
Fair Value Hedges | Pay fixed interest rate swaps with counterparty | Accrued interest and other liabilities
Derivatives, Fair Value [Line Items]
Notional Amount	20,310	21,301
Estimate Fair Value Gain	0	0
Estimate Fair Value Loss	(2,108)	(2,302)
Fair Value Hedges | Matched interest rate swaps | Accrued interest and other liabilities
Derivatives, Fair Value [Line Items]
Notional Amount	341,488	278,829
Estimate Fair Value Gain	29	131
Estimate Fair Value Loss	(17,521)	(15,502)
Fair Value Hedges | Matched interest rate swaps | Accrued interest and other assets
Derivatives, Fair Value [Line Items]
Notional Amount	341,488	278,829
Estimate Fair Value Gain	16,758	14,843
Estimate Fair Value Loss	(29)	(131)
Pay fixed interest rate swaps with counterparty
Derivatives, Fair Value [Line Items]
Notional Amount	$ 20,310