Mortgage Backed Securities Mortgage backed securities designated as AFS (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	$ 3,254,946	$ 3,181,586
Investments, Unamortized Premium (Discount), Net	758,950	800,080
Available-for-sale Securities, Amortized Cost Basis	4,013,896	3,981,666
Available-for-sale Debt Securities Gross Unrealized Gain	45,114	104,869
Available-for-sale Debt Securities, Gross Unrealized Loss	(69,566)	(12,623)
Mortgage-backed Securities Available-for-sale, Fair Value	3,989,444	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	2,865,398	2,706,428
Investments, Unamortized Premium (Discount), Net	627,047	709,246
Available-for-sale Securities, Amortized Cost Basis	3,492,445	3,415,674
Available-for-sale Debt Securities Gross Unrealized Gain	26,792	59,265
Available-for-sale Debt Securities, Gross Unrealized Loss	(63,976)	(12,349)
Mortgage-backed Securities Available-for-sale, Fair Value	3,455,261	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	2,591,568	2,425,826
Investments, Unamortized Premium (Discount), Net	154,220	137,168
Available-for-sale Securities, Amortized Cost Basis	2,745,788	2,562,994
Available-for-sale Debt Securities Gross Unrealized Gain	6,104	19,477
Available-for-sale Debt Securities, Gross Unrealized Loss	(59,742)	(11,134)
Mortgage-backed Securities Available-for-sale, Fair Value	2,692,150	2,571,337
Investments, Weighted Average Coupon Rate	3.22%	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	273,830	280,602
Investments, Unamortized Premium (Discount), Net	19,061	21,907
Available-for-sale Securities, Amortized Cost Basis	292,891	302,509
Available-for-sale Debt Securities Gross Unrealized Gain	10,793	21,570
Available-for-sale Debt Securities, Gross Unrealized Loss	(900)	(6)
Mortgage-backed Securities Available-for-sale, Fair Value	302,784	324,073
Investments, Weighted Average Coupon Rate	5.07%	5.19%
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Interest-Only-Strip [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	0	0
Notional balance for interest only securities	10,160,502	2,393,614
Investments, Unamortized Premium (Discount), Net	453,766	550,171
Available-for-sale Securities, Amortized Cost Basis	453,766	550,171
Available-for-sale Debt Securities Gross Unrealized Gain	9,895	18,218
Available-for-sale Debt Securities, Gross Unrealized Loss	(3,334)	(1,209)
Mortgage-backed Securities Available-for-sale, Fair Value	460,327	567,180
Investments, Weighted Average Coupon Rate	0.83%	0.95%
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	389,548	475,158
Investments, Unamortized Premium (Discount), Net	131,903	90,834
Available-for-sale Securities, Amortized Cost Basis	521,451	565,992
Available-for-sale Debt Securities Gross Unrealized Gain	18,322	45,604
Available-for-sale Debt Securities, Gross Unrealized Loss	(5,590)	(274)
Mortgage-backed Securities Available-for-sale, Fair Value	534,183	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	13,845	11,411
Investments, Unamortized Premium (Discount), Net	(338)	(781)
Available-for-sale Securities, Amortized Cost Basis	13,507	10,630
Available-for-sale Debt Securities Gross Unrealized Gain	338	606
Available-for-sale Debt Securities, Gross Unrealized Loss	(80)	(198)
Mortgage-backed Securities Available-for-sale, Fair Value	13,765	11,038
Investments, Weighted Average Coupon Rate	4.61%	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	375,703	463,747
Investments, Unamortized Premium (Discount), Net	(18,277)	(17,313)
Available-for-sale Securities, Amortized Cost Basis	357,426	446,434
Available-for-sale Debt Securities Gross Unrealized Gain	15,366	39,944
Available-for-sale Debt Securities, Gross Unrealized Loss	(3,511)	(36)
Mortgage-backed Securities Available-for-sale, Fair Value	369,281	486,342
Investments, Weighted Average Coupon Rate	5.10%	5.31%
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Interest-Only-Strip [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, Balance, Principal Amount	0	0
Notional balance for interest only securities	4,274,957	10,059,495
Investments, Unamortized Premium (Discount), Net	150,518	108,928
Available-for-sale Securities, Amortized Cost Basis	150,518	108,928
Available-for-sale Debt Securities Gross Unrealized Gain	2,618	5,054
Available-for-sale Debt Securities, Gross Unrealized Loss	(1,999)	(40)
Mortgage-backed Securities Available-for-sale, Fair Value	151,137	113,942
Investments, Weighted Average Coupon Rate	0.66%	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	1,912,937	1,026,277
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	(43,543)	(6,552)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	150	83
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	670,402	271,719
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	(20,433)	(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	162,558	13,877
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	(5,435)	(76)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	39	3
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	6,310	2,701
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ (155)	$ (198)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	6	8