Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Financial assets	$ 14,360	$ 2,246
Financial Instruments, Financial assets	(13,867)	(2,243)
Collateral, Financial assets
Net Amount, Financial assets	493	3
Net Amount Recognized, Resell agreements, Financial assets	4,898	4,898
Financial Instruments, Resell agreements, Financial assets
Collateral, Resell agreements, Financial assets	(4,898)	(4,898)
Net Amount, Resell agreements, Financial assets
Net Amount Recognized, Financial assets	19,258	7,144
Financial Instruments, Financial assets	(13,867)	(2,243)
Collateral, Financial assets	(4,898)	(4,898)
Net Amount, Financial assets	493	3
Net Amount Recognized, Financial liabilities	49,137	74,159
Financial Instruments, Financial liabilities	(13,867)	(2,243)
Collateral, Financial liabilities	(31,094)	(68,773)
Net Amount, Financial liabilities	4,176	3,143
Net Amount Recognized, Repurchase agreements, Financial liabilities	549,073	559,461
Financial Instruments, Repurchase agreements, Financial liabilities
Collateral, Repurchase agreements, Financial liabilities	(549,073)	(559,461)
Net Amount, Repurchase agreements, Financial liabilities
Net Amount Recognized, Financial liabilities	598,210	633,620
Financial Instruments, Financial liabilities	(13,867)	(2,243)
Collateral, Financial liabilities	(580,167)	(628,234)
Net Amount, Financial liabilities	4,176	3,143
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Financial assets	2,425	4
Financial Instruments, Financial assets	(2,425)	(4)
Collateral, Financial assets
Net Amount, Financial assets
Net Amount Recognized, Financial liabilities	23,412	33,999
Financial Instruments, Financial liabilities	(2,425)	(4)
Collateral, Financial liabilities	(19,708)	(33,778)
Net Amount, Financial liabilities	1,279	217
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Financial assets	6,045	2,033
Financial Instruments, Financial assets	(6,045)	(2,033)
Collateral, Financial assets
Net Amount, Financial assets
Net Amount Recognized, Financial liabilities	7,458	14,374
Financial Instruments, Financial liabilities	(6,045)	(2,033)
Collateral, Financial liabilities		(11,318)
Net Amount, Financial liabilities	1,413	1,023
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Financial assets	3,437	189
Financial Instruments, Financial assets	(3,437)	(189)
Collateral, Financial assets
Net Amount, Financial assets
Net Amount Recognized, Financial liabilities	11,888	13,807
Financial Instruments, Financial liabilities	(3,437)	(189)
Collateral, Financial liabilities	(8,451)	(13,618)
Net Amount, Financial liabilities
Other Counterparties [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Financial assets	2,453	20
Financial Instruments, Financial assets	(1,960)	(17)
Collateral, Financial assets
Net Amount, Financial assets	493	3
Net Amount Recognized, Financial liabilities	6,379	11,979
Financial Instruments, Financial liabilities	(1,960)	(17)
Collateral, Financial liabilities	(2,935)	(10,059)
Net Amount, Financial liabilities	$ 1,484	$ 1,903