Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Financial assets	$ 2,424	$ 2,246	$ 6,818
Financial Instruments, Financial assets	(2,421)	(2,243)	(6,818)
Collateral, Financial assets
Net Amount, Financial assets	3	3
Net Amount Recognized, Resell agreements, Financial assets	4,898	4,898	8,052
Financial Instruments, Resell agreements, Financial assets
Collateral, Resell agreements, Financial assets	(4,898)	(4,898)	(8,052)
Net Amount, Resell agreements, Financial assets
Net Amount Recognized, Financial assets	7,322	7,144	14,870
Financial Instruments, Financial assets	(2,421)	(2,243)	(6,818)
Collateral, Financial assets	(4,898)	(4,898)	(8,052)
Net Amount, Financial assets	3	3
Net Amount Recognized, Financial liabilities	68,942	74,159	83,587
Financial Instruments, Financial liabilities	(2,421)	(2,243)	(6,818)
Collateral, Financial liabilities	(61,215)	(68,773)	(75,563)
Net Amount, Financial liabilities	5,306	3,143	1,206
Net Amount Recognized, Repurchase agreements, Financial liabilities	508,702	559,461	635,248
Financial Instruments, Repurchase agreements, Financial liabilities
Collateral, Repurchase agreements, Financial liabilities	(508,702)	(559,461)	(635,248)
Net Amount, Repurchase agreements, Financial liabilities
Net Amount Recognized, Financial liabilities	577,644	633,620	718,835
Financial Instruments, Financial liabilities	(2,421)	(2,243)	(6,818)
Collateral, Financial liabilities	(569,917)	(628,234)	(710,811)
Net Amount, Financial liabilities	5,306	3,143	1,206
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Financial assets	7	4	28
Financial Instruments, Financial assets	(7)	(4)	(28)
Collateral, Financial assets
Net Amount, Financial assets
Net Amount Recognized, Financial liabilities	30,656	33,999	35,481
Financial Instruments, Financial liabilities	(7)	(4)	(28)
Collateral, Financial liabilities	(29,830)	(33,778)	(35,453)
Net Amount, Financial liabilities	819	217
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Financial assets	671	2,033	6,655
Financial Instruments, Financial assets	(671)	(2,033)	(6,655)
Collateral, Financial assets
Net Amount, Financial assets
Net Amount Recognized, Financial liabilities	13,715	14,374	20,407
Financial Instruments, Financial liabilities	(671)	(2,033)	(6,655)
Collateral, Financial liabilities	(10,703)	(11,318)	(13,752)
Net Amount, Financial liabilities	2,341	1,023
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Financial assets	1,549	189	135
Financial Instruments, Financial assets	(1,549)	(189)	(135)
Collateral, Financial assets
Net Amount, Financial assets
Net Amount Recognized, Financial liabilities	14,561	13,807	14,436
Financial Instruments, Financial liabilities	(1,549)	(189)	(135)
Collateral, Financial liabilities	(13,012)	(13,618)	(14,301)
Other Counterparties [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Financial assets	197	20
Financial Instruments, Financial assets	(194)	(17)
Collateral, Financial assets
Net Amount, Financial assets	3	3
Net Amount Recognized, Financial liabilities	10,010	11,979	13,263
Financial Instruments, Financial liabilities	(194)	(17)
Collateral, Financial liabilities	(7,670)	(10,059)	(12,057)
Net Amount, Financial liabilities	$ 2,146	$ 1,903	$ 1,206