MBS (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
MBS
Minimum term of fixed rate mortgages underlying MBS	15 years
Information about MBS
Principal/ Current Face	$ 13,058,400,000	$ 13,302,328,000
Purchase Premiums	232,099,000	228,256,000
Accretable Purchase Discounts	(381,964,000)	(371,684,000)
Discount Designated as Credit Reserve and OTTI	(1,312,952,000)	(1,380,506,000)
Amortized Cost	11,599,295,000	11,782,817,000
Fair Value	12,542,182,000	12,607,625,000
Gross Unrealized Gains	951,367,000	832,560,000
Gross Unrealized Losses	(8,480,000)	(7,752,000)
Net Unrealized Gain/(Loss)	942,887,000	824,808,000	312,954,000	55,491,000
Credit Reserve	1,266,000,000	1,332,000,000
OTTI	46,900,000	48,700,000
Principal payments receivable	3,700,000	4,400,000
Agency MBS
Information about MBS
Principal/ Current Face	6,745,295,000	6,792,768,000
Purchase Premiums	231,297,000	227,384,000
Accretable Purchase Discounts	(51,000)	(58,000)
Amortized Cost	6,980,253,000	7,024,517,000
Fair Value	7,153,905,000	7,225,460,000
Gross Unrealized Gains	179,509,000	202,397,000
Gross Unrealized Losses	(5,857,000)	(1,454,000)
Net Unrealized Gain/(Loss)	173,652,000	200,943,000
Agency MBS | Fannie Mae.
Information about MBS
Principal/ Current Face	5,631,101,000	5,977,388,000
Purchase Premiums	188,186,000	196,686,000
Accretable Purchase Discounts	(51,000)	(58,000)
Amortized Cost	5,819,236,000	6,174,016,000
Fair Value	5,974,186,000	6,351,621,000
Gross Unrealized Gains	157,790,000	178,970,000
Gross Unrealized Losses	(2,840,000)	(1,365,000)
Net Unrealized Gain/(Loss)	154,950,000	177,605,000
Agency MBS | Freddie Mac.
Information about MBS
Principal/ Current Face	1,100,024,000	800,854,000
Purchase Premiums	42,866,000	30,447,000
Amortized Cost	1,146,602,000	835,724,000
Fair Value	1,164,812,000	858,560,000
Gross Unrealized Gains	21,227,000	22,925,000
Gross Unrealized Losses	(3,017,000)	(89,000)
Net Unrealized Gain/(Loss)	18,210,000	22,836,000
Agency MBS | Ginnie Mae.
Information about MBS
Principal/ Current Face	14,170,000	14,526,000
Purchase Premiums	245,000	251,000
Amortized Cost	14,415,000	14,777,000
Fair Value	14,907,000	15,279,000
Gross Unrealized Gains	492,000	502,000
Net Unrealized Gain/(Loss)	492,000	502,000
Non-Agency MBS
Information about MBS
Principal/ Current Face	6,313,105,000	6,509,560,000
Purchase Premiums	802,000	872,000
Accretable Purchase Discounts	(381,913,000)	(371,626,000)
Discount Designated as Credit Reserve and OTTI	(1,312,952,000)	(1,380,506,000)
Amortized Cost	4,619,042,000	4,758,300,000
Fair Value	5,388,277,000	5,382,165,000
Gross Unrealized Gains	771,858,000	630,163,000
Gross Unrealized Losses	(2,623,000)	(6,298,000)
Net Unrealized Gain/(Loss)	769,235,000	623,865,000
Non-Agency MBS | Rated AAA
Information about MBS
Principal/ Current Face	22,658,000	25,209,000
Purchase Premiums	135,000	158,000
Accretable Purchase Discounts	(205,000)	(219,000)
Amortized Cost	22,588,000	25,148,000
Fair Value	23,338,000	25,905,000
Gross Unrealized Gains	750,000	757,000
Net Unrealized Gain/(Loss)	750,000	757,000
Non-Agency MBS | Rated A
Information about MBS
Principal/ Current Face	1,008,000	1,147,000
Purchase Premiums	21,000	24,000
Amortized Cost	1,029,000	1,171,000
Fair Value	988,000	1,086,000
Gross Unrealized Losses	(41,000)	(85,000)
Net Unrealized Gain/(Loss)	(41,000)	(85,000)
Non-Agency MBS | Rated BBB
Information about MBS
Principal/ Current Face	42,818,000	49,301,000
Purchase Premiums	580,000	637,000
Accretable Purchase Discounts	(1,346,000)	(1,741,000)
Discount Designated as Credit Reserve and OTTI		(378,000)
Amortized Cost	42,052,000	47,819,000
Fair Value	43,440,000	48,563,000
Gross Unrealized Gains	2,079,000	1,806,000
Gross Unrealized Losses	(691,000)	(1,062,000)
Net Unrealized Gain/(Loss)	1,388,000	744,000
Non-Agency MBS | Rated BB
Information about MBS
Principal/ Current Face	118,881,000	118,031,000
Purchase Premiums	53,000	39,000
Accretable Purchase Discounts	(9,028,000)	(8,892,000)
Discount Designated as Credit Reserve and OTTI	(874,000)	(853,000)
Amortized Cost	109,032,000	108,325,000
Fair Value	116,887,000	112,905,000
Gross Unrealized Gains	8,088,000	4,937,000
Gross Unrealized Losses	(233,000)	(357,000)
Net Unrealized Gain/(Loss)	7,855,000	4,580,000
Non-Agency MBS | Rated B
Information about MBS
Principal/ Current Face	241,825,000	247,532,000
Accretable Purchase Discounts	(31,771,000)	(31,133,000)
Discount Designated as Credit Reserve and OTTI	(10,377,000)	(12,462,000)
Amortized Cost	199,677,000	203,937,000
Fair Value	228,401,000	225,281,000
Gross Unrealized Gains	28,724,000	21,452,000
Gross Unrealized Losses		(108,000)
Net Unrealized Gain/(Loss)	28,724,000	21,344,000
Non-Agency MBS | Rated CCC
Information about MBS
Principal/ Current Face	1,205,624,000	1,235,638,000
Purchase Premiums	13,000	14,000
Accretable Purchase Discounts	(115,858,000)	(107,618,000)
Discount Designated as Credit Reserve and OTTI	(188,857,000)	(201,126,000)
Amortized Cost	900,922,000	926,908,000
Fair Value	1,063,379,000	1,055,757,000
Gross Unrealized Gains	163,933,000	131,826,000
Gross Unrealized Losses	(1,476,000)	(2,977,000)
Net Unrealized Gain/(Loss)	162,457,000	128,849,000
Non-Agency MBS | Rated CC
Information about MBS
Principal/ Current Face	554,897,000	579,632,000
Accretable Purchase Discounts	(36,537,000)	(41,191,000)
Discount Designated as Credit Reserve and OTTI	(132,586,000)	(132,061,000)
Amortized Cost	385,774,000	406,380,000
Fair Value	459,565,000	468,017,000
Gross Unrealized Gains	73,791,000	61,739,000
Gross Unrealized Losses		(102,000)
Net Unrealized Gain/(Loss)	73,791,000	61,637,000
Non-Agency MBS | Rated C
Information about MBS
Principal/ Current Face	733,338,000	952,984,000
Accretable Purchase Discounts	(47,516,000)	(55,294,000)
Discount Designated as Credit Reserve and OTTI	(126,533,000)	(166,529,000)
Amortized Cost	559,289,000	731,161,000
Fair Value	639,835,000	812,523,000
Gross Unrealized Gains	80,568,000	81,850,000
Gross Unrealized Losses	(22,000)	(488,000)
Net Unrealized Gain/(Loss)	80,546,000	81,362,000
Non-Agency MBS | Unrated and other
Information about MBS
Principal/ Current Face	3,392,056,000	3,300,086,000
Accretable Purchase Discounts	(139,652,000)	(125,538,000)
Discount Designated as Credit Reserve and OTTI	(853,725,000)	(867,097,000)
Amortized Cost	2,398,679,000	2,307,451,000
Fair Value	2,812,444,000	2,632,128,000
Gross Unrealized Gains	413,925,000	325,796,000
Gross Unrealized Losses	(160,000)	(1,119,000)
Net Unrealized Gain/(Loss)	413,765,000	324,677,000
Non-Agency MBS | D-rated
Information about MBS
Amortized Cost	2,348,000,000	2,252,000,000
Fair Value	$ 2,754,000,000	$ 2,573,000,000
Number of mortgage backed securities held	268	246