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, 2014	Dec. 31, 2013
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	$ 12,745	$ 16,265
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(12,264)	(14,824)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(212)	(1,360)
Net Amount, Derivatives, Financial Assets	269	81
Net Amount Recognized, Resell agreements	4,898	7,898
Gross Amounts Not Offset Financial Instruments, Resell Agreements	0	0
Gross Amounts Not Offset Collateral, Resell Agreements, Financial Assets	(4,898)	(7,898)
Net Amount, Financial Assets, Resell Agreements	0	0
Net Amount Recognized, Total Financial Assets	17,643	24,163
Gross Amounts Not Offset Collateral, Financial Assets	(5,110)	(9,258)
Net Amount, Financial Assets	269	81
Net Amount Recognized, Derivatives, Financial Liabilities	39,551	42,603
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(12,264)	(14,824)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(26,534)	(27,012)
Net Amount, Derivatives, Financial Liabilities	753	767
Net Amount Recognized, Repurchase Agreements, Financial Liabilities	576,040	668,053
Gross Amounts Not Offset Financial Instruments, Repurchase Agreements, Financial Liabilities	0	0
Gross Amounts Not Offset Collateral, Repurchase Agreements, Financial Liabilities	(576,040)	(668,053)
Net Amount, Repurchase Agreements, Financial Liabilities	0	0
Net Amount Recognized, Financial Liabilities	615,591	710,656
Gross Amounts Not Offset Collateral, Financial Liabilities	(602,574)	(695,065)
Net Amount, Financial Liabilities	753	767
Counterparty A [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	1,493	3,342
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(1,493)	(3,342)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	17,875	18,615
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(1,493)	(3,342)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(15,943)	(15,167)
Net Amount, Derivatives, Financial Liabilities	439	106
Counterparty B [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	5,140	8,196
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(5,140)	(8,196)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	7,146	9,054
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(5,140)	(8,196)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(2,006)	(613)
Net Amount, Derivatives, Financial Liabilities	0	245
Counterparty C [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Net Amount Recognized, Derivatives, Financial Assets	2,157	1,187
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(2,157)	(1,187)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	0	0
Net Amount, Derivatives, Financial Assets	0	0
Net Amount Recognized, Derivatives, Financial Liabilities	7,440	10,870
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(2,157)	(1,187)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(5,283)	(9,683)
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	3,955	3,540
Gross Amounts Not Offset, Financial Instruments, Total Derivatives, Financial Assets	(3,474)	(2,099)
Gross Amount Not Offset Collateral, Derivatives, Financial Assets	(212)	(1,360)
Net Amount, Derivatives, Financial Assets	269	81
Net Amount Recognized, Derivatives, Financial Liabilities	7,090	4,064
Gross Amounts Not Offset Financial Instruments, Total Derivatives, Financial Liabilities	(3,474)	(2,099)
Gross Amounts Not Offset Collateral, Derivatives, Financial Liabilities	(3,302)	(1,549)
Net Amount, Derivatives, Financial Liabilities	$ 314	$ 416