Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 11,978	$ 14,543
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(9,377)	(10,825)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(2,601)	(2,725)
Net Amount, Derivatives, Financial Assets	0	993
Net Amount Recognized, Resell agreements, Financial Assets	11,642	13,442
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(11,642)	(13,442)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	23,620	27,985
Gross Amounts Not Offset Collateral, Financial Assets	(14,243)	(16,167)
Net Amount, Financial Assets	0	993
Net Amount Recognized, Derivatives, Financial Liabilities	57,551	40,198
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(9,377)	(10,825)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(45,670)	(28,803)
Net Amount, Derivatives, Financial Liabilities	2,504	570
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	685,036	883,947
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(685,036)	(883,947)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	742,587	924,145
Gross Amounts Not Offset Collateral, Financial Liabilities	(730,706)	(912,750)
Net Amount, Financial Liabilities	2,504	570
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	324	743
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(324)	(743)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	20,135	16,092
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(324)	(743)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(19,588)	(15,347)
Net Amount, Derivatives, Financial Liabilities	223	2
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	5,227	6,901
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(5,227)	(5,795)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	(413)
Net Amount, Derivatives, Financial Assets	0	693
Net Amount Recognized, Derivatives, Financial Liabilities	7,853	5,795
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(5,227)	(5,795)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(1,458)	0
Net Amount, Derivatives, Financial Liabilities	1,168	0
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	6,286	6,059
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(3,685)	(3,447)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(2,601)	(2,312)
Net Amount, Derivatives, Financial Assets	0	300
Net Amount Recognized, Derivatives, Financial Liabilities	3,685	3,447
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(3,685)	(3,447)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	0	0
Net Amount, Derivatives, Financial Liabilities	0	0
Other Counterparties [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	141	840
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(141)	(840)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	25,878	14,864
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(141)	(840)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(24,624)	(13,456)
Net Amount, Derivatives, Financial Liabilities	$ 1,113	$ 568