Balance Sheet Offsetting (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative Financial Assets, Gross Amount Recognized	$ 27,607	$ 25,915
Derivative Financial Liabilities, Gross Amounts Recognized	58,146	72,126
Offsetting Of Financial Assets [Member] | Dealer A [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	1,823	0
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Net Amount	1,823	0
Offsetting Of Financial Assets [Member] | Dealer B [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	2,648	873
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	(510)	0
Net Amount	2,138	873
Offsetting Of Financial Assets [Member] | Dealer C [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	332	5
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Net Amount	332	5
Offsetting Of Financial Assets [Member] | Dealer D [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	4,525	0
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	(2,930)	0
Net Amount	1,595	0
Offsetting Of Financial Assets [Member] | Dealer E [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	0	0
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Net Amount	0	0
Offsetting Of Financial Assets [Member] | Dealer F [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	0	0
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Net Amount	0	0
Offsetting Of Financial Assets [Member] | Dealer G [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	0	0
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Net Amount	0	0
Offsetting Of Financial Assets [Member] | Dealer H [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	0	0
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Net Amount	0	0
Offsetting Of Financial Assets [Member] | End User [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	18,279	25,037
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Net Amount	18,279	25,037
Offsetting Of Financial Assets [Member] | Counterparty Total [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	27,607	25,915
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	(3,440)	0
Net Amount	24,167	25,915
Offsetting Of Financial Assets [Member] | Derivative Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Gross Amount Recognized	27,612	25,922
Derivative Financial Assets, Gross Amounts Offset	(5)	(7)
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	27,607	25,915
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	(3,440)	0
Net Amount	24,167	25,915
Offsetting Of Financial Liabilities [Member] | Dealer A [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	5,028	7,789
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	(2,920)	(7,820)
Net Amount	2,108	(31)
Offsetting Of Financial Liabilities [Member] | Dealer B [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	3,107	6,104
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	(5,050)
Net Amount	3,107	1,054
Offsetting Of Financial Liabilities [Member] | Dealer C [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	1,375	3,460
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	(900)	(3,300)
Net Amount	475	160
Offsetting Of Financial Liabilities [Member] | Dealer D [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	1,218	3,239
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	(3,350)
Net Amount	1,218	(111)
Offsetting Of Financial Liabilities [Member] | Dealer E [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	25,144	33,598
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	(26,500)	(35,800)
Net Amount	(1,356)	(2,202)
Offsetting Of Financial Liabilities [Member] | Dealer F [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	6,870	9,105
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	(8,200)	(10,300)
Net Amount	(1,330)	(1,195)
Offsetting Of Financial Liabilities [Member] | Dealer G [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	5,091	6,623
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	(5,610)	(7,150)
Net Amount	(519)	(527)
Offsetting Of Financial Liabilities [Member] | Dealer H [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	923	1,816
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	(860)	(1,820)
Net Amount	63	(4)
Offsetting Of Financial Liabilities [Member] | End User [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	9,390	392
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	(251)	(513)
Net Amount	9,139	(121)
Offsetting Of Financial Liabilities [Member] | Counterparty Total [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	58,146	72,126
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	(45,241)	(75,103)
Net Amount	12,905	(2,977)
Offsetting Of Financial Liabilities [Member] | Derivative Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Gross Amounts Recognized	58,146	72,126
Derivative Financial Liabilities, Gross Amounts Offset	0	0
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	58,146	72,126
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	(45,241)	(75,103)
Net Amount	12,905	(2,977)
Offsetting Of Financial Liabilities [Member] | Repurchase Agreements [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Gross Amounts Recognized	296,625	302,577
Derivative Financial Liabilities, Gross Amounts Offset	0	0
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	296,625	302,577
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	(296,625)	(302,577)
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	0	0
Net Amount	0	0
Offsetting Of Financial Liabilities [Member] | Total Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Gross Amounts Recognized	354,771	374,703
Derivative Financial Liabilities, Gross Amounts Offset	0	0
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	354,771	374,703
Financial Instruments, Gross Amounts Not Offset In The Consolidated Balance Sheet	(296,625)	(302,577)
Cash Collateral, Gross Amounts Not Offset In The Consolidated Balance Sheet	(45,241)	(75,103)
Net Amount	$ 12,905	$ (2,977)