Balance Sheet Offsetting (Derivative Assets And Derivative Liabilities) (Detail) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	$ 24,586,000	$ 24,316,000
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	43,164,000	50,247,000
Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	24,586,000	24,316,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	(1,250,000)	(3,940,000)
Derivative Financial Assets, Net Amount	23,336,000	20,376,000
Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	43,164,000	50,247,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(43,826,000)	(42,813,000)
Derivative Financial Liabilities, Net Amount	(662,000)	7,434,000
Clearing Member A [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	0	19,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	0	19,000
Clearing Member A [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	6,641,000	1,077,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(9,992,000)	(3,993,000)
Derivative Financial Liabilities, Net Amount	(3,351,000)	(2,916,000)
Cleared Agreements [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	0	19,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	0	19,000
Cleared Agreements [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	6,641,000	1,077,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(9,992,000)	(3,993,000)
Derivative Financial Liabilities, Net Amount	(3,351,000)	(2,916,000)
Dealer A [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	774,000	2,836,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	774,000	2,836,000
Dealer A [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	3,869,000	3,840,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(3,120,000)	(1,290,000)
Derivative Financial Liabilities, Net Amount	749,000	2,550,000
Dealer B [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	965,000	2,412,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	965,000	2,412,000
Dealer B [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	3,305,000	3,306,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(2,040,000)	(1,400,000)
Derivative Financial Liabilities, Net Amount	1,265,000	1,906,000
Dealer C [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	54,000	359,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	54,000	359,000
Dealer C [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	1,181,000	684,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(1,150,000)	(550,000)
Derivative Financial Liabilities, Net Amount	31,000	134,000
Dealer D [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	2,345,000	5,571,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	(1,250,000)	(3,940,000)
Derivative Financial Assets, Net Amount	1,095,000	1,631,000
Dealer D [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	1,265,000	688,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	0	0
Derivative Financial Liabilities, Net Amount	1,265,000	688,000
Dealer E [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	0	0
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	0	0
Dealer E [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	16,261,000	21,251,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(18,600,000)	(24,100,000)
Derivative Financial Liabilities, Net Amount	(2,339,000)	(2,849,000)
Dealer F [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	0	0
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	0	0
Dealer F [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	3,080,000	5,081,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(4,200,000)	(6,100,000)
Derivative Financial Liabilities, Net Amount	(1,120,000)	(1,019,000)
Dealer G [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	0	0
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	0	0
Dealer G [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	2,664,000	3,966,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(3,130,000)	(4,260,000)
Derivative Financial Liabilities, Net Amount	(466,000)	(294,000)
Dealer H [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	0	0
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	0	0
Dealer H [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	876,000	770,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(1,060,000)	(1,120,000)
Derivative Financial Liabilities, Net Amount	(184,000)	(350,000)
Dealer I [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	0	0
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	0	0
Dealer I [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	400,000	121,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(280,000)	0
Derivative Financial Liabilities, Net Amount	120,000	121,000
Dealer J [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	0	0
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	0	0
Dealer J [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	18,000	18,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	0	0
Derivative Financial Liabilities, Net Amount	18,000	18,000
End User [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	20,448,000	13,119,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	20,448,000	13,119,000
End User [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	3,604,000	9,445,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(254,000)	0
Derivative Financial Liabilities, Net Amount	3,350,000	9,445,000
Bilateral Agreements Counterparty [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	24,586,000	24,297,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	(1,250,000)	(3,940,000)
Derivative Financial Assets, Net Amount	23,336,000	20,357,000
Bilateral Agreements Counterparty [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	36,523,000	49,170,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(33,834,000)	(38,820,000)
Derivative Financial Liabilities, Net Amount	2,689,000	10,350,000
Counterparty [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	24,586,000	24,316,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	(1,250,000)	(3,940,000)
Derivative Financial Assets, Net Amount	23,336,000	20,376,000
Counterparty [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	43,164,000	50,247,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(43,826,000)	(42,813,000)
Derivative Financial Liabilities, Net Amount	$ (662,000)	$ 7,434,000