MBS and CRT Securities (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Information about MBS and CRT securities
Minimum term of fixed rate mortgages underlying MBS	15 years
Principal/ Current Face	$ 11,039,694,000
Purchase Premiums	213,790,000
Accretable Purchase Discounts	(404,362,000)
Discount Designated as Credit Reserve and OTTI	(900,557,000)
Amortized Cost	9,949,107,000
Fair Value	10,762,622,000
Gross Unrealized Gains	852,963,000
Gross Unrealized Losses	(39,448,000)
Net Unrealized Gain/(Loss)	813,515,000
Principal payments receivable	542,000	1,100,000
Total MBS
Information about MBS and CRT securities
Principal/ Current Face	10,930,194,000	11,892,957,000
Purchase Premiums	213,790,000	227,533,000
Accretable Purchase Discounts	(399,635,000)	(460,126,000)
Discount Designated as Credit Reserve and OTTI	(900,557,000)	(1,043,037,000)
Amortized Cost	9,844,334,000	10,618,446,000
Fair Value	10,659,639,000	11,371,358,000
Gross Unrealized Gains	852,639,000	848,946,000
Gross Unrealized Losses	(37,334,000)	(96,034,000)
Net Unrealized Gain/(Loss)	815,305,000	752,912,000
Agency MBS
Information about MBS and CRT securities
Principal/ Current Face	5,610,293,000	6,276,919,000
Purchase Premiums	213,329,000	226,895,000
Accretable Purchase Discounts	(71,000)	(87,000)
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	5,824,093,000	6,504,846,000
Fair Value	5,904,207,000	6,519,221,000
Gross Unrealized Gains	113,736,000	106,660,000
Gross Unrealized Losses	(33,622,000)	(92,285,000)
Net Unrealized Gain/(Loss)	80,114,000	14,375,000
Agency MBS | Fannie Mae
Information about MBS and CRT securities
Principal/ Current Face	4,587,823,000	5,092,410,000
Purchase Premiums	174,245,000	181,710,000
Accretable Purchase Discounts	(71,000)	(87,000)
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	4,761,997,000	5,274,033,000
Fair Value	4,843,084,000	5,315,363,000
Gross Unrealized Gains	102,187,000	96,516,000
Gross Unrealized Losses	(21,100,000)	(55,186,000)
Net Unrealized Gain/(Loss)	81,087,000	41,330,000
Agency MBS | Freddie Mac
Information about MBS and CRT securities
Principal/ Current Face	1,011,659,000	1,171,841,000
Purchase Premiums	38,895,000	44,967,000
Accretable Purchase Discounts	0	0
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	1,051,096,000	1,217,927,000
Fair Value	1,049,854,000	1,190,670,000
Gross Unrealized Gains	11,280,000	9,842,000
Gross Unrealized Losses	(12,522,000)	(37,099,000)
Net Unrealized Gain/(Loss)	(1,242,000)	(27,257,000)
Agency MBS | Ginnie Mae
Information about MBS and CRT securities
Principal/ Current Face	10,811,000	12,668,000
Purchase Premiums	189,000	218,000
Accretable Purchase Discounts	0	0
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	11,000,000	12,886,000
Fair Value	11,269,000	13,188,000
Gross Unrealized Gains	269,000	302,000
Gross Unrealized Losses	0	0
Net Unrealized Gain/(Loss)	269,000	302,000
Non-Agency MBS
Information about MBS and CRT securities
Principal/ Current Face	5,319,901,000	5,616,038,000
Purchase Premiums	461,000	638,000
Accretable Purchase Discounts	(399,564,000)	(460,039,000)
Discount Designated as Credit Reserve and OTTI	(900,557,000)	(1,043,037,000)
Amortized Cost	4,020,241,000	4,113,600,000
Fair Value	4,755,432,000	4,852,137,000
Gross Unrealized Gains	738,903,000	742,286,000
Gross Unrealized Losses	(3,712,000)	(3,749,000)
Net Unrealized Gain/(Loss)	735,191,000	738,537,000
Credit Reserve	877,600,000	998,500,000
OTTI	23,000,000	44,500,000
Non-Agency MBS | Expected to Recover Par
Information about MBS and CRT securities
Principal/ Current Face	431,788,000	234,187,000
Purchase Premiums	461,000	638,000
Accretable Purchase Discounts	(29,501,000)	(24,450,000)
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	402,748,000	210,375,000
Fair Value	428,431,000	230,738,000
Gross Unrealized Gains	26,735,000	21,720,000
Gross Unrealized Losses	(1,052,000)	(1,357,000)
Net Unrealized Gain/(Loss)	25,683,000	20,363,000
Non-Agency MBS | Expected to Recover Less Than Par
Information about MBS and CRT securities
Principal/ Current Face	4,888,113,000	5,381,851,000
Purchase Premiums	0	0
Accretable Purchase Discounts	(370,063,000)	(435,589,000)
Discount Designated as Credit Reserve and OTTI	(900,557,000)	(1,043,037,000)
Amortized Cost	3,617,493,000	3,903,225,000
Fair Value	4,327,001,000	4,621,399,000
Gross Unrealized Gains	712,168,000	720,566,000
Gross Unrealized Losses	(2,660,000)	(2,392,000)
Net Unrealized Gain/(Loss)	709,508,000	718,174,000
Current face amount of non-Agency MBS expected to be recovered (percent)	83.00%	81.00%
CRT securities
Information about MBS and CRT securities
Principal/ Current Face	109,500,000
Purchase Premiums	0
Accretable Purchase Discounts	(4,727,000)
Discount Designated as Credit Reserve and OTTI	0
Amortized Cost	104,773,000
Fair Value	102,983,000	0
Gross Unrealized Gains	324,000
Gross Unrealized Losses	(2,114,000)
Net Unrealized Gain/(Loss)	(1,790,000)
RPL/NPL MBS
Information about MBS and CRT securities
Principal/ Current Face	161,000,000	3,900,000
Amortized Cost	161,000,000
Fair Value	$ 161,000,000