Balance Sheet Offsetting (Derivative Assets And Derivative Liabilities) (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, 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, 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
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	6,485,000	1,077,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(6,485,000)	(1,077,000)
Derivative Financial Liabilities, Net Amount	0	0
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	6,485,000	1,077,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(6,485,000)	(1,077,000)
Derivative Financial Liabilities, Net Amount	0	0
Dealer A [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	1,054,000	2,836,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	1,054,000	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	3,470,000	3,840,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(2,560,000)	(1,290,000)
Derivative Financial Liabilities, Net Amount	910,000	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	1,145,000	2,412,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	1,145,000	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	2,039,000	3,306,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(1,310,000)	(1,400,000)
Derivative Financial Liabilities, Net Amount	729,000	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	129,000	359,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	129,000	359,000
Dealer C [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	1,002,000	684,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(850,000)	(550,000)
Derivative Financial Liabilities, Net Amount	152,000	134,000
Dealer D [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	2,722,000	5,571,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	(1,590,000)	(3,940,000)
Derivative Financial Assets, Net Amount	1,132,000	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	953,000	688,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	0	0
Derivative Financial Liabilities, Net Amount	953,000	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	12,642,000	21,251,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(12,642,000)	(21,251,000)
Derivative Financial Liabilities, Net Amount	0	0
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	2,049,000	5,081,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(2,049,000)	(5,081,000)
Derivative Financial Liabilities, Net Amount	0	0
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	2,014,000	3,966,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(2,014,000)	(3,966,000)
Derivative Financial Liabilities, Net Amount	0	0
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	668,000	770,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(668,000)	(770,000)
Derivative Financial Liabilities, Net Amount	0	0
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	394,000	121,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(280,000)	0
Derivative Financial Liabilities, Net Amount	114,000	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	13,000	18,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	0	0
Derivative Financial Liabilities, Net Amount	13,000	18,000
End User [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
Derivative Financial Assets, Net Amounts Presented In The Consolidated Balance Sheet	17,289,000	13,119,000
Derivative Financial Assets, Financial Instruments	0	0
Derivative Financial Assets, Cash Collateral	0	0
Derivative Financial Assets, Net Amount	17,289,000	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	3,939,000	9,445,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	0	0
Derivative Financial Liabilities, Net Amount	3,939,000	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	22,339,000	24,297,000
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	20,357,000
Bilateral Agreements Counterparty [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
Derivative Financial Liabilities, Net Amounts Presented In The Consolidated Balance Sheet	29,183,000	49,170,000
Derivative Financial Liabilities, Financial Instruments	0	0
Derivative Financial Liabilities, Cash Collateral	(22,373,000)	(34,308,000)
Derivative Financial Liabilities, Net Amount	6,810,000	14,862,000
Counterparty [Member] | Offsetting Of Financial Assets [Member]
Derivative [Line Items]
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
Counterparty [Member] | Offsetting Of Financial Liabilities [Member]
Derivative [Line Items]
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