Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Derivatives: Net Amount Recognized	$ 12,458	$ 2,246
Gross Amounts Not Offset, Financial Instruments	(12,074)	(2,243)
Collateral, Financial assets	(384)	0
Financial Assets, Net Amount	0	3
Net Amount Recognized, Resell agreements	4,898	4,898
Financial Instruments, Resell Agreements	0	0
Collateral, Resell agreements, Financial assets	(4,898)	(4,898)
Net Amount, Financial Assets, Resell agreements	0	0
Net Amount Recognized, Total Financial assets	17,356	7,144
Collateral, Financial assets	(5,282)	(4,898)
Net Amount, Financial assets	0	3
Net Amount Recognized, Derivatives, Financial Liabilities	48,374	74,159
Financial Instruments, Total derivatives, Financial liabilities	(12,074)	(2,243)
Collateral, Derivatives, Financial liabilities	(32,631)	(68,773)
Net Amount, Derivatives, Financial liabilities	3,669	3,143
Net Amount Recognized, Repurchase agreements, Financial liabilities	584,612	559,461
Financial Instruments, Repurchase agreements, Financial liabilities	0	0
Collateral, Repurchase agreements, Financial liabilities	(584,612)	(559,461)
Net Amount, Repurchase agreements, Financial liabilities	0	0
Net Amount Recognized, Financial liabilities	632,986	633,620
Collateral, Financial liabilities	(617,243)	(628,234)
Net Amount, Financial liabilities	3,669	3,143
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Derivatives: Net Amount Recognized	2,384	4
Gross Amounts Not Offset, Financial Instruments	(2,384)	(4)
Collateral, Financial assets	0	0
Financial Assets, Net Amount	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	21,496	33,999
Financial Instruments, Total derivatives, Financial liabilities	(2,384)	(4)
Collateral, Derivatives, Financial liabilities	(18,754)	(33,778)
Net Amount, Derivatives, Financial liabilities	358	217
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Derivatives: Net Amount Recognized	5,069	2,033
Gross Amounts Not Offset, Financial Instruments	(5,069)	(2,033)
Collateral, Financial assets	0	0
Financial Assets, Net Amount	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	9,736	14,374
Financial Instruments, Total derivatives, Financial liabilities	(5,069)	(2,033)
Collateral, Derivatives, Financial liabilities	(2,940)	(11,318)
Net Amount, Derivatives, Financial liabilities	1,727	1,023
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Derivatives: Net Amount Recognized	2,830	189
Gross Amounts Not Offset, Financial Instruments	(2,830)	(189)
Collateral, Financial assets	0	0
Financial Assets, Net Amount	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	12,061	13,807
Financial Instruments, Total derivatives, Financial liabilities	(2,830)	(189)
Collateral, Derivatives, Financial liabilities	(9,231)	(13,618)
Net Amount, Derivatives, Financial liabilities	0	0
Other Counterparties [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Derivatives: Net Amount Recognized	2,175	20
Gross Amounts Not Offset, Financial Instruments	(1,791)	(17)
Collateral, Financial assets	(384)	0
Financial Assets, Net Amount	0	3
Net Amount Recognized, Derivatives, Financial Liabilities	5,081	11,979
Financial Instruments, Total derivatives, Financial liabilities	(1,791)	(17)
Collateral, Derivatives, Financial liabilities	(1,706)	(10,059)
Net Amount, Derivatives, Financial liabilities	$ 1,584	$ 1,903