Balance Sheet Offsetting - Financial Instruments Derivative Assets Liabilities and Resell Agreements Net of Amount Not Offset (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 17,029	$ 29,921
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(13,947)	(20,436)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(2,581)	(6,035)
Net Amount, Derivatives, Financial Assets	501	3,450
Net Amount Recognized, Resell agreements	13,642	9,642
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(13,642)	(9,642)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	30,671	39,563
Gross Amounts Not Offset Collateral, Financial Assets	(16,223)	(15,677)
Net Amount, Financial Assets	501	3,450
Net Amount Recognized, Derivatives, Financial Liabilities	40,331	47,898
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(13,947)	(20,436)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(25,202)	(26,307)
Net Amount, Derivatives, Financial Liabilities	1,182	1,155
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	580,714	791,119
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(580,714)	(791,119)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	621,045	839,017
Gross Amounts Not Offset Collateral, Financial Liabilities	(605,916)	(817,426)
Net Amount, Financial Liabilities	1,182	1,155
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	1,047	627
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,047)	(627)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	16,561	18,653
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,047)	(627)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(14,750)	(17,626)
Net Amount, Derivatives, Financial Liabilities	764	400
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	10,019	17,308
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(7,715)	(9,506)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(1,805)	(4,925)
Net Amount, Derivatives, Financial Assets	499	2,877
Net Amount Recognized, Derivatives, Financial Liabilities	7,715	9,506
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(7,715)	(9,506)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	0	0
Net Amount, Derivatives, Financial Liabilities	0	0
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	2,915	7,991
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(2,915)	(7,539)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	452
Net Amount Recognized, Derivatives, Financial Liabilities	5,159	7,539
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(2,915)	(7,539)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(2,244)	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	3,048	3,995
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(2,270)	(2,764)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(776)	(1,110)
Net Amount, Derivatives, Financial Assets	2	121
Net Amount Recognized, Derivatives, Financial Liabilities	10,896	12,200
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(2,270)	(2,764)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(8,208)	(8,681)
Net Amount, Derivatives, Financial Liabilities	$ 418	$ 755