MBS (Details) (USD $)	9 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010
MBS
Minimum term of fixed rate mortgages underlying MBS	15 years
Information about MBS
Current Face	$ 13,445,304,000		$ 12,155,624,000
Purchase Premiums	226,358,000		178,904,000
Accretable Purchase Discounts	(308,146,000)		(250,614,000)
Discount Designated as Credit Reserve and OTTI	(1,459,651,000)		(1,228,766,000)
Amortized Cost	11,909,205,000		10,857,486,000
Fair Value	12,673,823,000		10,912,977,000
Gross Unrealized Gains	778,784,000		345,200,000
Gross Unrealized Losses	(14,166,000)		(289,709,000)
Net Unrealized Gain/(Loss)	764,618,000	296,007,000	55,491,000	194,583,000	292,376,000	393,822,000
Credit Reserve	1,409,000,000		1,174,000,000
OTTI	50,300,000		54,500,000
Principal payments receivable	5,300,000		2,300,000
Agency MBS
Information about MBS
Current Face	6,988,233,000		6,741,271,000
Purchase Premiums	225,449,000		177,801,000
Accretable Purchase Discounts	(70,000)		(135,000)
Amortized Cost	7,218,952,000		6,921,275,000
Fair Value	7,476,848,000		7,137,531,000
Gross Unrealized Gains	258,857,000		218,000,000
Gross Unrealized Losses	(961,000)		(1,744,000)
Net Unrealized Gain/(Loss)	257,896,000		216,256,000
Agency MBS | Fannie Mae.
Information about MBS
Current Face	6,357,355,000		5,981,834,000
Purchase Premiums	204,196,000		154,809,000
Accretable Purchase Discounts	(70,000)		(135,000)
Amortized Cost	6,561,481,000		6,136,508,000
Fair Value	6,792,421,000		6,329,925,000
Gross Unrealized Gains	231,889,000		194,997,000
Gross Unrealized Losses	(949,000)		(1,580,000)
Net Unrealized Gain/(Loss)	230,940,000		193,417,000
Agency MBS | Freddie Mac.
Information about MBS
Current Face	615,976,000		743,517,000
Purchase Premiums	20,996,000		22,717,000
Amortized Cost	642,312,000		768,572,000
Fair Value	668,748,000		791,085,000
Gross Unrealized Gains	26,448,000		22,677,000
Gross Unrealized Losses	(12,000)		(164,000)
Net Unrealized Gain/(Loss)	26,436,000		22,513,000
Agency MBS | Ginnie Mae.
Information about MBS
Current Face	14,902,000		15,920,000
Purchase Premiums	257,000		275,000
Amortized Cost	15,159,000		16,195,000
Fair Value	15,679,000		16,521,000
Gross Unrealized Gains	520,000		326,000
Net Unrealized Gain/(Loss)	520,000		326,000
Non-Agency MBS
Information about MBS
Current Face	6,457,071,000		5,414,353,000
Purchase Premiums	909,000		1,103,000
Accretable Purchase Discounts	(308,076,000)		(250,479,000)
Discount Designated as Credit Reserve and OTTI	(1,459,651,000)		(1,228,766,000)
Amortized Cost	4,690,253,000		3,936,211,000
Fair Value	5,196,975,000		3,775,446,000
Gross Unrealized Gains	519,927,000		127,200,000
Gross Unrealized Losses	(13,205,000)		(287,965,000)
Net Unrealized Gain/(Loss)	506,722,000		(160,765,000)
Non-Agency MBS | Rated AAA
Information about MBS
Current Face	35,908,000		12,258,000
Purchase Premiums	179,000		245,000
Accretable Purchase Discounts	(938,000)
Amortized Cost	35,149,000		12,503,000
Fair Value	36,259,000		12,258,000
Gross Unrealized Gains	1,110,000
Gross Unrealized Losses			(245,000)
Net Unrealized Gain/(Loss)	1,110,000		(245,000)
Non-Agency MBS | Rated AA
Information about MBS
Current Face	46,000		47,000
Purchase Premiums	1,000		1,000
Amortized Cost	47,000		48,000
Fair Value	39,000		34,000
Gross Unrealized Losses	(8,000)		(14,000)
Net Unrealized Gain/(Loss)	(8,000)		(14,000)
Non-Agency MBS | Rated A
Information about MBS
Current Face	22,713,000		28,950,000
Purchase Premiums	644,000		765,000
Accretable Purchase Discounts			(624,000)
Discount Designated as Credit Reserve and OTTI			(5,000)
Amortized Cost	23,357,000		29,086,000
Fair Value	22,214,000		24,911,000
Gross Unrealized Gains			341,000
Gross Unrealized Losses	(1,143,000)		(4,516,000)
Net Unrealized Gain/(Loss)	(1,143,000)		(4,175,000)
Non-Agency MBS | Rated BBB
Information about MBS
Current Face	32,049,000		46,593,000
Purchase Premiums	29,000		42,000
Accretable Purchase Discounts	(2,123,000)		(3,020,000)
Discount Designated as Credit Reserve and OTTI	(378,000)		(582,000)
Amortized Cost	29,577,000		43,033,000
Fair Value	31,391,000		38,352,000
Gross Unrealized Gains	1,981,000
Gross Unrealized Losses	(167,000)		(4,681,000)
Net Unrealized Gain/(Loss)	1,814,000		(4,681,000)
Non-Agency MBS | Rated BB
Information about MBS
Current Face	120,010,000		100,513,000
Purchase Premiums	41,000		33,000
Accretable Purchase Discounts	(8,802,000)		(10,749,000)
Discount Designated as Credit Reserve and OTTI	(1,724,000)		(3,223,000)
Amortized Cost	109,525,000		86,574,000
Fair Value	110,928,000		81,789,000
Gross Unrealized Gains	2,456,000		2,232,000
Gross Unrealized Losses	(1,053,000)		(7,017,000)
Net Unrealized Gain/(Loss)	1,403,000		(4,785,000)
Non-Agency MBS | Rated B
Information about MBS
Current Face	307,488,000		355,930,000
Purchase Premiums	15,000		17,000
Accretable Purchase Discounts	(27,539,000)		(30,584,000)
Discount Designated as Credit Reserve and OTTI	(18,879,000)		(25,004,000)
Amortized Cost	261,085,000		300,359,000
Fair Value	275,878,000		277,438,000
Gross Unrealized Gains	17,036,000		2,729,000
Gross Unrealized Losses	(2,243,000)		(25,650,000)
Net Unrealized Gain/(Loss)	14,793,000		(22,921,000)
Non-Agency MBS | Rated CCC
Information about MBS
Current Face	1,155,928,000		1,031,407,000
Accretable Purchase Discounts	(86,864,000)		(68,174,000)
Discount Designated as Credit Reserve and OTTI	(203,508,000)		(203,185,000)
Amortized Cost	865,556,000		760,048,000
Fair Value	969,810,000		741,028,000
Gross Unrealized Gains	107,609,000		27,767,000
Gross Unrealized Losses	(3,355,000)		(46,787,000)
Net Unrealized Gain/(Loss)	104,254,000		(19,020,000)
Non-Agency MBS | Rated CC
Information about MBS
Current Face	647,499,000		687,664,000
Accretable Purchase Discounts	(27,799,000)		(33,478,000)
Discount Designated as Credit Reserve and OTTI	(128,181,000)		(142,777,000)
Amortized Cost	491,519,000		511,409,000
Fair Value	533,770,000		487,619,000
Gross Unrealized Gains	42,923,000		14,209,000
Gross Unrealized Losses	(672,000)		(37,999,000)
Net Unrealized Gain/(Loss)	42,251,000		(23,790,000)
Non-Agency MBS | Rated C
Information about MBS
Current Face	1,104,962,000		2,128,919,000
Accretable Purchase Discounts	(44,439,000)		(64,963,000)
Discount Designated as Credit Reserve and OTTI	(225,212,000)		(487,397,000)
Amortized Cost	835,311,000		1,576,559,000
Fair Value	916,709,000		1,503,737,000
Gross Unrealized Gains	83,075,000		44,988,000
Gross Unrealized Losses	(1,677,000)		(117,810,000)
Net Unrealized Gain/(Loss)	81,398,000		(72,822,000)
Non-Agency MBS | Unrated and other
Information about MBS
Current Face	3,030,468,000		1,022,072,000
Accretable Purchase Discounts	(109,572,000)		(38,887,000)
Discount Designated as Credit Reserve and OTTI	(881,769,000)		(366,593,000)
Amortized Cost	2,039,127,000		616,592,000
Fair Value	2,299,977,000		608,280,000
Gross Unrealized Gains	263,737,000		34,934,000
Gross Unrealized Losses	(2,887,000)		(43,246,000)
Net Unrealized Gain/(Loss)	260,850,000		(8,312,000)
Non-Agency MBS | D-rated
Information about MBS
Amortized Cost	2,025,000,000		602,000,000
Fair Value	$ 2,281,000,000		$ 593,800,000
Number of mortgage backed securities held	214		78