Mortgage Backed Securities Mortgage backed securities designated as AFS (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	$ 3,709,044	$ 3,181,586
Investments, Unamortized Premium (Discount), Net	841,787	800,080
Available-for-sale Securities, Amortized Cost Basis	4,550,831	3,981,666
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(20,542)	92,246
Mortgage-backed Securities Available-for-sale, Fair Value	4,530,289	4,073,912
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	3,247,756	2,706,428
Investments, Unamortized Premium (Discount), Net	732,147	709,246
Available-for-sale Securities, Amortized Cost Basis	3,979,903	3,415,674
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(32,071)	46,916
Mortgage-backed Securities Available-for-sale, Fair Value	3,947,832	3,462,590
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	2,966,661	2,425,826
Investments, Unamortized Premium (Discount), Net	170,225	137,168
Available-for-sale Securities, Amortized Cost Basis	3,136,886	2,562,994
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(52,519)	8,343
Mortgage-backed Securities Available-for-sale, Fair Value	3,084,367	2,571,337
Investments, Weighted Average Coupon Rate	3.34%	3.67%
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	281,095	280,602
Investments, Unamortized Premium (Discount), Net	21,438	21,907
Available-for-sale Securities, Amortized Cost Basis	302,533	302,509
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	10,743	21,564
Mortgage-backed Securities Available-for-sale, Fair Value	313,276	324,073
Investments, Weighted Average Coupon Rate	5.20%	5.19%
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Commercial Mortgage Backed Securities, Interest Only [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	0	0
Notional balance for interest only securities	10,641,141	10,059,495
Investments, Unamortized Premium (Discount), Net	540,484	550,171
Available-for-sale Securities, Amortized Cost Basis	540,484	550,171
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	9,705	17,009
Mortgage-backed Securities Available-for-sale, Fair Value	550,189	567,180
Investments, Weighted Average Coupon Rate	0.87%	0.95%
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	461,288	475,158
Investments, Unamortized Premium (Discount), Net	109,640	90,834
Available-for-sale Securities, Amortized Cost Basis	570,928	565,992
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	11,529	45,330
Mortgage-backed Securities Available-for-sale, Fair Value	582,457	611,322
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	12,315	11,411
Investments, Unamortized Premium (Discount), Net	(352)	(781)
Available-for-sale Securities, Amortized Cost Basis	11,963	10,630
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	101	408
Mortgage-backed Securities Available-for-sale, Fair Value	12,064	11,038
Investments, Weighted Average Coupon Rate	4.64%	4.28%
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	448,973	463,747
Investments, Unamortized Premium (Discount), Net	(18,436)	(17,313)
Available-for-sale Securities, Amortized Cost Basis	430,537	446,434
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	9,238	39,908
Mortgage-backed Securities Available-for-sale, Fair Value	439,775	486,342
Investments, Weighted Average Coupon Rate	5.33%	5.31%
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Commercial Mortgage Backed Securities, Interest Only [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	0	0
Notional balance for interest only securities	2,821,042	2,393,614
Investments, Unamortized Premium (Discount), Net	128,428	108,928
Available-for-sale Securities, Amortized Cost Basis	128,428	108,928
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	2,190	5,014
Mortgage-backed Securities Available-for-sale, Fair Value	130,618	113,942
Investments, Weighted Average Coupon Rate	0.87%	0.86%
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,687,758	1,026,277
Available-for-sale Securities, Continuous Loss Position Less Than 12 Months, Aggregate Losses	(54,626)	(6,552)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	212	83
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	255,924	271,719
Available-for-sale Securities, Continuous Unrealized Loss Position for 12 Months or Longer, Aggregate Losses	(8,915)	(5,797)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	44	34
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	148,060	13,877
Available-for-sale Securities, Continuous Loss Position Less Than 12 Months, Aggregate Losses	(7,684)	(76)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	22	3
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,996	2,701
Available-for-sale Securities, Continuous Unrealized Loss Position for 12 Months or Longer, Aggregate Losses	$ (156)	$ (198)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	6	8