Derivative Instruments Derivative Call Options, Notional Amount and Fair Value, by Counterparty (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Derivative, Collateral, Obligation to Return Cash	$ 328,700,000	$ 165,400,000
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	93,700,000	109,300,000
Not Designated as Hedging Instrument [Member] | Call Option [Member]
Derivative [Line Items]
Derivative instruments	415,258,000	273,314,000
Derivative, Notional Amount	15,623,505,000	14,069,330,000
Not Designated as Hedging Instrument [Member] | Call Option [Member] | Derivative Counterparty, Bank of America [Member]
Derivative [Line Items]
Derivative instruments	16,533,000	43,481,000
Derivative, Notional Amount	568,786,000	2,340,213,000
Not Designated as Hedging Instrument [Member] | Call Option [Member] | Derivative Counterparty, Barclays [Member]
Derivative [Line Items]
Derivative instruments	103,929,000	60,903,000
Derivative, Notional Amount	3,463,777,000	2,419,339,000
Not Designated as Hedging Instrument [Member] | Call Option [Member] | Derivative Counterparty, BNP Paribas [Member]
Derivative [Line Items]
Derivative instruments	60,301,000	48,293,000
Derivative, Notional Amount	2,207,097,000	2,533,301,000
Not Designated as Hedging Instrument [Member] | Call Option [Member] | Derivative Counterparty, Citibank, N.A. [Member] [Member]
Derivative [Line Items]
Derivative instruments	67,592,000	0
Derivative, Notional Amount	2,878,588,000	0
Not Designated as Hedging Instrument [Member] | Call Option [Member] | Derivative Counterparty, Credit Suisse [Member]
Derivative [Line Items]
Derivative instruments	21,518,000	27,464,000
Derivative, Notional Amount	936,625,000	1,423,802,000
Not Designated as Hedging Instrument [Member] | Call Option [Member] | Derivative Counterparty, Deutsche Bank [Member]
Derivative [Line Items]
Derivative instruments	20,787,000	7,697,000
Derivative, Notional Amount	886,688,000	384,420,000
Not Designated as Hedging Instrument [Member] | Call Option [Member] | Derivative Counterparty, HSBC [Member]
Derivative [Line Items]
Derivative instruments	6,539,000	4,557,000
Derivative, Notional Amount	295,520,000	348,674,000
Not Designated as Hedging Instrument [Member] | Call Option [Member] | Derivative Counterparty, J.P. Morgan [Member]
Derivative [Line Items]
Derivative instruments	21,940,000	27,961,000
Derivative, Notional Amount	735,016,000	2,109,019,000
Not Designated as Hedging Instrument [Member] | Call Option [Member] | Derivative Counterparty, Morgan Stanley [Member]
Derivative [Line Items]
Derivative instruments	40,113,000	7,375,000
Derivative, Notional Amount	1,590,505,000	244,180,000
Not Designated as Hedging Instrument [Member] | Call Option [Member] | Derivative Counterparty, UBS [Member]
Derivative [Line Items]
Derivative instruments	0	240,000
Derivative, Notional Amount	0	39,147,000
Not Designated as Hedging Instrument [Member] | Call Option [Member] | Derivative Counterparty, Wells Fargo (Wachovia) [Member]
Derivative [Line Items]
Derivative instruments	56,006,000	45,343,000
Derivative, Notional Amount	$ 2,060,903,000	$ 2,227,235,000