Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 3,451	$ 14,543
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(3,451)	(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	14,142	13,442
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(14,142)	(13,442)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	17,593	27,985
Gross Amounts Not Offset Collateral, Financial Assets	(14,142)	(16,167)
Net Amount, Financial Assets	0	993
Net Amount Recognized, Derivatives, Financial Liabilities	68,640	40,198
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(3,451)	(10,825)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(62,897)	(28,803)
Net Amount, Derivatives, Financial Liabilities	2,292	570
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	720,860	883,947
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(720,860)	(883,947)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	789,500	924,145
Gross Amounts Not Offset Collateral, Financial Liabilities	(783,757)	(912,750)
Net Amount, Financial Liabilities	2,292	570
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	196	743
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(196)	(743)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	20,942	16,092
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(196)	(743)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(20,746)	(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	2,139	6,901
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(2,139)	(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	10,815	5,795
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(2,139)	(5,795)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(7,776)	0
Net Amount, Derivatives, Financial Liabilities	900	0
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	1,038	6,059
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,038)	(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	4,501	3,447
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,038)	(3,447)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(3,002)	0
Net Amount, Derivatives, Financial Liabilities	461	0
Other Counterparties [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	78	840
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(78)	(840)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	32,382	14,864
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(78)	(840)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(31,373)	(13,456)
Net Amount, Derivatives, Financial Liabilities	$ 931	$ 568