Balance Sheet Offsetting (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	$ 22,339,000	$ 24,316,000
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	35,668,000	50,247,000
Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Gross Amounts Recognized	22,339,000	24,316,000
Derivative Financial Assets, Gross Amounts Offset	0	0
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	22,339,000	24,316,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	(1,590,000)	(3,940,000)
Derivative Financial Assets, Net Amount	20,749,000	20,376,000
Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Gross Amounts Recognized	35,668,000	50,247,000
Derivative Financial Liabilities, Gross Amounts Offset	0	0
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	35,668,000	50,247,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(28,858,000)	(35,385,000)
Derivative Financial Liabilities, Net Amount	6,810,000	14,862,000
Repurchase Agreements, Gross Amounts Recognized	281,085,000	295,800,000
Repurchase Agreements, Gross Amounts Offset	0	0
Repurchase Agreements, Net Amounts Presented In The Consolidated Balance Sheet	281,085,000	295,800,000
Repurchase Agreements, Financial Instruments	(281,085,000)	(295,800,000)
Repurchase Agreements, Cash Collateral	0	0
Repurchase Agreements, Net Amount	0	0
Financial Liabilities, Gross Amounts Recognized	316,753,000	346,047,000
Financial Liabilities, Gross Amounts Offset	0	0
Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	316,753,000	346,047,000
Financial Liabilities, Financial Instruments	(281,085,000)	(295,800,000)
Financial Liabilities, Cash Collateral	(28,858,000)	(35,385,000)
Financial Liabilities, Net Amount	$ 6,810,000	$ 14,862,000