Balance Sheet Offsetting (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative Financial Assets, Gross Amounts Recognized	$ 22,936,000	$ 25,915,000
Derivative Financial Liabilities, Gross Amounts Recognized	54,712,000	72,126,000
Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Gross Amounts Recognized	22,939,000	25,922,000
Derivative Financial Assets, Gross Amounts Offset	(3,000)	(7,000)
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	22,936,000	25,915,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	(2,720,000)	0
Derivative Financial Assets, Net Amount	20,216,000	25,915,000
Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Gross Amounts Recognized	54,712,000	72,126,000
Derivative Financial Liabilities, Gross Amounts Offset	0	0
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	54,712,000	72,126,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(44,828,000)	(75,103,000)
Derivative Financial Liabilities, Net Amount	9,884,000	(2,977,000)
Repurchase Agreements, Gross Amounts Recognized	310,736,000	302,577,000
Repurchase Agreements, Gross Amounts Offset	0	0
Repurchase Agreements, Net Amounts Presented In The Consolidated Balance Sheet	310,736,000	302,577,000
Repurchase Agreements, Financial Instruments	(310,736,000)	(302,577,000)
Repurchase Agreements, Cash Collateral	0	0
Repurchase Agreements, Net Amount	0	0
Financial Liabilities, Gross Amounts Recognized	365,448,000	374,703,000
Financial Liabilities, Gross Amounts Offset	0	0
Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	365,448,000	374,703,000
Financial Liabilities, Financial Instruments	(310,736,000)	(302,577,000)
Financial Liabilities, Cash Collateral	(44,828,000)	(75,103,000)
Financial Liabilities, Net Amount	$ 9,884,000	$ (2,977,000)