Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 1,981	$ 14,543
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,981)	(10,825)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	(2,725)
Net Amount, Derivatives, Financial Assets	0	993
Net Amount Recognized, Resell agreements, Financial Assets	9,642	13,442
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(9,642)	(13,442)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	11,623	27,985
Gross Amounts Not Offset Collateral, Financial Assets	(9,642)	(16,167)
Net Amount, Financial Assets	0	993
Net Amount Recognized, Derivatives, Financial Liabilities	59,728	40,198
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,981)	(10,825)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(55,506)	(28,803)
Net Amount, Derivatives, Financial Liabilities	2,241	570
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	793,702	883,947
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(793,702)	(883,947)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	853,430	924,145
Gross Amounts Not Offset Collateral, Financial Liabilities	(849,208)	(912,750)
Net Amount, Financial Liabilities	2,241	570
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	169	743
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(169)	(743)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	18,517	16,092
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(169)	(743)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(18,348)	(15,347)
Net Amount, Derivatives, Financial Liabilities	0	2
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	496	6,901
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(496)	(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	8,810	5,795
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(496)	(5,795)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(7,331)	0
Net Amount, Derivatives, Financial Liabilities	983	0
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	1,242	6,059
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,242)	(3,447)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	(2,312)
Net Amount, Derivatives, Financial Assets	0	300
Net Amount Recognized, Derivatives, Financial Liabilities	3,571	3,447
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,242)	(3,447)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(1,746)	0
Net Amount, Derivatives, Financial Liabilities	583	0
Counterparty D [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Liabilities	24,038	11,505
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	0	(20)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(24,038)	(11,485)
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	74	840
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(74)	(840)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	4,792	3,359
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(74)	(820)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(4,043)	(1,971)
Net Amount, Derivatives, Financial Liabilities	$ 675	$ 568