Derivative Financial Instruments (Financial Assets and Liabilities for Derivative Positions, Summarized by Counterparty) (Detail) (USD $)	Dec. 31, 2013		Dec. 31, 2012
Derivative [Line Items]
Total MTM(Loss) Gain	$ (66,000)		$ (43,173,000)
Cash Collateral Posted (Received)	8,115,000		43,444,000
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount	13,138,667,000		8,003,512,000
MTM Gain	6,581,000		0
MTM Loss	(622,000)		(1,627,000)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
MTM Gain	17,062,000		544,000
MTM Loss	(23,087,000)		(42,090,000)
Dealer A [Member]
Derivative [Line Items]
Notional Amount	387,258,000		561,716,000
Total MTM(Loss) Gain	(4,274,000)		(17,507,000)
Cash Collateral Posted (Received)	4,300,000		17,900,000
Net Exposure	26,000	[1]	393,000	[1]
Dealer A [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
MTM Gain	730,000		0
MTM Loss	0		(985,000)
Dealer A [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
MTM Gain	4,643,000		199,000
MTM Loss	(9,647,000)		(16,721,000)
Dealer B [Member]
Derivative [Line Items]
Notional Amount	322,888,000		403,097,000
Total MTM(Loss) Gain	(5,010,000)		(15,784,000)
Cash Collateral Posted (Received)	4,940,000		16,980,000
Net Exposure	0	[1]	1,196,000	[1]
Dealer B [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
MTM Gain	615,000		0
MTM Loss	0		(642,000)
Dealer B [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
MTM Gain	3,475,000		139,000
MTM Loss	(9,100,000)		(15,281,000)
Dealer C [Member]
Derivative [Line Items]
Notional Amount	14,477,000		15,221,000
Total MTM(Loss) Gain	(1,348,000)		(2,037,000)
Cash Collateral Posted (Received)	0		0
Net Exposure	0	[1]	0	[1]
Dealer C [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
MTM Gain	0		0
MTM Loss	0		0
Dealer C [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
MTM Gain	0		1,000
MTM Loss	(1,348,000)		(2,038,000)
Dealer D [Member]
Derivative [Line Items]
Notional Amount	291,627,000		184,648,000
Total MTM(Loss) Gain	5,250,000		(2,453,000)
Derivative, Collateral, Right to Reclaim Cash	(5,300,000)
Cash Collateral Posted (Received)			2,600,000
Net Exposure	0	[1]	147,000	[1]
Dealer D [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
MTM Gain	1,734,000		0
MTM Loss	0		0
Dealer D [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
MTM Gain	4,108,000		53,000
MTM Loss	(592,000)		(2,506,000)
Dealer E [Member]
Derivative [Line Items]
Notional Amount	372,771,000		238,830,000
Total MTM(Loss) Gain	3,549,000		(5,267,000)
Derivative, Collateral, Right to Reclaim Cash	(3,310,000)
Cash Collateral Posted (Received)			5,290,000
Net Exposure	0	[1]	23,000	[1]
Dealer E [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
MTM Gain	2,290,000		0
MTM Loss	(15,000)		0
Dealer E [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
MTM Gain	3,017,000		152,000
MTM Loss	(1,743,000)		(5,419,000)
Dealer F [Member]
Derivative [Line Items]
Notional Amount	11,749,646,000		6,600,000,000
Total MTM(Loss) Gain	1,767,000		(125,000)
Cash Collateral Posted (Received)	7,485,000		674,000
Net Exposure	9,252,000	[1]	549,000	[1]
Dealer F [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
MTM Gain	1,212,000
MTM Loss	(607,000)
Dealer F [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
MTM Gain	1,819,000
MTM Loss	$ (657,000)		$ (125,000)

[1] Net positive exposure represents over-collateralized loss positions which can be the result of OTC clearing house initial margin requirements posted in compliance with the Dodd-Frank Wall Street Reform and Consumer Protection Act ("Dodd-Frank").