Mortgage Backed Securities Mortgage backed securities designated as AFS (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	$ 3,380,838	$ 3,181,586
Investments, Unamortized Premium (Discount), Net	754,316	800,080
Available-for-sale Securities, Amortized Cost Basis	4,135,154	3,981,666
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(22,388)	92,246
Mortgage-backed Securities Available-for-sale, Fair Value	4,112,766	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	2,984,524	2,706,428
Investments, Unamortized Premium (Discount), Net	648,708	709,246
Available-for-sale Securities, Amortized Cost Basis	3,633,232	3,415,674
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(35,352)	46,916
Mortgage-backed Securities Available-for-sale, Fair Value	3,597,880	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	2,723,084	2,425,826
Investments, Unamortized Premium (Discount), Net	162,370	137,168
Available-for-sale Securities, Amortized Cost Basis	2,885,454	2,562,994
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(56,407)	8,343
Mortgage-backed Securities Available-for-sale, Fair Value	2,829,047	2,571,337
Investments, Weighted Average Coupon Rate	3.26%	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	261,440	280,602
Investments, Unamortized Premium (Discount), Net	19,844	21,907
Available-for-sale Securities, Amortized Cost Basis	281,284	302,509
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	11,941	21,564
Mortgage-backed Securities Available-for-sale, Fair Value	293,225	324,073
Investments, Weighted Average Coupon Rate	4.82%	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	9,682,065	10,059,495
Investments, Unamortized Premium (Discount), Net	466,494	550,171
Available-for-sale Securities, Amortized Cost Basis	466,494	550,171
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	9,114	17,009
Mortgage-backed Securities Available-for-sale, Fair Value	475,608	567,180
Investments, Weighted Average Coupon Rate	0.89%	0.95%
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	396,314	475,158
Investments, Unamortized Premium (Discount), Net	105,608	90,834
Available-for-sale Securities, Amortized Cost Basis	501,922	565,992
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	12,964	45,330
Mortgage-backed Securities Available-for-sale, Fair Value	514,886	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	14,972	11,411
Investments, Unamortized Premium (Discount), Net	(351)	(781)
Available-for-sale Securities, Amortized Cost Basis	14,621	10,630
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(33)	408
Mortgage-backed Securities Available-for-sale, Fair Value	14,588	11,038
Investments, Weighted Average Coupon Rate	4.58%	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	381,342	463,747
Investments, Unamortized Premium (Discount), Net	(17,213)	(17,313)
Available-for-sale Securities, Amortized Cost Basis	364,129	446,434
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	10,620	39,908
Mortgage-backed Securities Available-for-sale, Fair Value	374,749	486,342
Investments, Weighted Average Coupon Rate	5.52%	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	2,857,343	2,393,614
Investments, Unamortized Premium (Discount), Net	123,172	108,928
Available-for-sale Securities, Amortized Cost Basis	123,172	108,928
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	2,377	5,014
Mortgage-backed Securities Available-for-sale, Fair Value	125,549	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	2,129,694	1,026,277
Available-for-sale Securities, Continuous Loss Position Less Than 12 Months, Aggregate Losses	(48,337)	(6,552)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	164	83
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	558,070	271,719
Available-for-sale Securities, Continuous Unrealized Loss Position for 12 Months or Longer, Aggregate Losses	(17,718)	(5,797)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	67	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	126,846	13,877
Available-for-sale Securities, Continuous Loss Position Less Than 12 Months, Aggregate Losses	(5,902)	(76)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	23	3
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,820	2,701
Available-for-sale Securities, Continuous Unrealized Loss Position for 12 Months or Longer, Aggregate Losses	$ (100)	$ (198)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	6	8