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Residential Mortgage Securities and MSR Related Assets (MBS and CRT Securities) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Principal/ Current Face	$ 4,044,818,000	$ 6,646,178,000
Purchase Premiums	67,108,000	112,369,000
Accretable Purchase Discounts	(90,694,000)	(154,942,000)
Discount Designated as Credit Reserve and OTTI	(436,598,000)	(516,116,000)
Amortized Cost	3,585,248,000	6,088,525,000
Fair Value	3,983,519,000	6,509,333,000
Gross Unrealized Gains	417,089,000	484,591,000
Gross Unrealized Losses	(18,818,000)	(63,783,000)
Net Unrealized Gain/(Loss)	398,271,000	420,808,000
Principal payments receivable	614,000	1,000,000.0
Agency MBS
Debt Securities, Available-for-sale [Line Items]
Principal/ Current Face	1,604,583,000	2,630,630,000
Purchase Premiums	62,790,000	103,008,000
Accretable Purchase Discounts	(22,000)	(24,000)
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	1,667,965,000	2,734,650,000
Fair Value	1,664,582,000	2,698,213,000
Gross Unrealized Gains	15,326,000	13,061,000
Gross Unrealized Losses	(18,709,000)	(49,498,000)
Net Unrealized Gain/(Loss)	(3,383,000)	(36,437,000)
Agency MBS | Fannie Mae
Debt Securities, Available-for-sale [Line Items]
Principal/ Current Face	1,119,708,000	1,716,340,000
Purchase Premiums	43,249,000	65,930,000
Accretable Purchase Discounts	(22,000)	(24,000)
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	1,162,935,000	1,782,246,000
Fair Value	1,157,993,000	1,762,032,000
Gross Unrealized Gains	9,799,000	12,107,000
Gross Unrealized Losses	(14,741,000)	(32,321,000)
Net Unrealized Gain/(Loss)	(4,942,000)	(20,214,000)
Agency MBS | Freddie Mac
Debt Securities, Available-for-sale [Line Items]
Principal/ Current Face	480,879,000	909,561,000
Purchase Premiums	19,468,000	36,991,000
Accretable Purchase Discounts	0	0
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	500,961,000	947,588,000
Fair Value	502,468,000	931,318,000
Gross Unrealized Gains	5,475,000	907,000
Gross Unrealized Losses	(3,968,000)	(17,177,000)
Net Unrealized Gain/(Loss)	1,507,000	(16,270,000)
Agency MBS | Ginnie Mae
Debt Securities, Available-for-sale [Line Items]
Principal/ Current Face	3,996,000	4,729,000
Purchase Premiums	73,000	87,000
Accretable Purchase Discounts	0	0
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	4,069,000	4,816,000
Fair Value	4,121,000	4,863,000
Gross Unrealized Gains	52,000	47,000
Gross Unrealized Losses	0	0
Net Unrealized Gain/(Loss)	52,000	47,000
Non-Agency MBS
Debt Securities, Available-for-sale [Line Items]
Principal/ Current Face	2,195,303,000	3,538,804,000
Purchase Premiums	0	40,000
Accretable Purchase Discounts	(90,617,000)	(155,025,000)
Discount Designated as Credit Reserve and OTTI	(436,598,000)	(516,116,000)
Amortized Cost	1,668,088,000	2,867,703,000
Fair Value	2,063,529,000	3,318,299,000
Gross Unrealized Gains	395,459,000	458,985,000
Gross Unrealized Losses	(18,000)	(8,389,000)
Net Unrealized Gain/(Loss)	395,441,000	450,596,000
Credit reserve	426,000,000.0	503,300,000
OTTI	10,600,000	12,800,000
Expected to Recover Par
Debt Securities, Available-for-sale [Line Items]
Principal/ Current Face	722,477,000	1,536,485,000
Purchase Premiums	0	40,000
Accretable Purchase Discounts	(16,661,000)	(21,725,000)
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	705,816,000	1,514,800,000
Fair Value	725,668,000	1,527,700,000
Gross Unrealized Gains	19,861,000	20,520,000
Gross Unrealized Losses	(9,000)	(7,620,000)
Net Unrealized Gain/(Loss)	19,852,000	12,900,000
Expected to Recover Less Than Par
Debt Securities, Available-for-sale [Line Items]
Principal/ Current Face	1,472,826,000	2,002,319,000
Purchase Premiums	0	0
Accretable Purchase Discounts	(73,956,000)	(133,300,000)
Discount Designated as Credit Reserve and OTTI	(436,598,000)	(516,116,000)
Amortized Cost	962,272,000	1,352,903,000
Fair Value	1,337,861,000	1,790,599,000
Gross Unrealized Gains	375,598,000	438,465,000
Gross Unrealized Losses	(9,000)	(769,000)
Net Unrealized Gain/(Loss)	$ 375,589,000	$ 437,696,000
Percentage of current face amount of Non-Agency MBS to be recovered	80.00%	85.00%
Total MBS
Debt Securities, Available-for-sale [Line Items]
Principal/ Current Face	$ 3,799,886,000	$ 6,169,434,000
Purchase Premiums	62,790,000	103,048,000
Accretable Purchase Discounts	(90,639,000)	(155,049,000)
Discount Designated as Credit Reserve and OTTI	(436,598,000)	(516,116,000)
Amortized Cost	3,336,053,000	5,602,353,000
Fair Value	3,728,111,000	6,016,512,000
Gross Unrealized Gains	410,785,000	472,046,000
Gross Unrealized Losses	(18,727,000)	(57,887,000)
Net Unrealized Gain/(Loss)	392,058,000	414,159,000
CRT securities
Debt Securities, Available-for-sale [Line Items]
Principal/ Current Face	244,932,000	476,744,000
Purchase Premiums	4,318,000	9,321,000
Accretable Purchase Discounts	(55,000)	107,000
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	249,195,000	486,172,000
Fair Value	255,408,000	492,821,000
Gross Unrealized Gains	6,304,000	12,545,000
Gross Unrealized Losses	(91,000)	(5,896,000)
Net Unrealized Gain/(Loss)	6,213,000	6,649,000
Agency MBS, Fair Value Option
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	4,500,000	0
Gross Unrealized Losses	0	(3,300,000)
Debt securities, fair value option	280,300,000	736,500,000
RPL/NPL MBS
Debt Securities, Available-for-sale [Line Items]
Principal/ Current Face	632,300,000	1,400,000,000
Amortized Cost	631,800,000	1,400,000,000
Fair Value	635,000,000.0	1,400,000,000
CRT, Fair Value Option
Debt Securities, Available-for-sale [Line Items]
Fair Value	255,400,000	477,400,000
Gross Unrealized Gains	6,300,000	12,500,000
Gross Unrealized Losses	$ (91,000)	$ (5,600,000)

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