Mortgage-Backed Securities and other securities (Details 5) $ in Thousands	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Fair Value
Fair value - Less than 12 months	$ 1,419,604	$ 806,717
12 Months or More Fair Value	123,064	288,928
Total Fair Value	1,542,668	1,095,645
Unrealized Losses
Less than 12 Months Unrealized Losses	(13,832)	(18,182)
12 Months or More Unrealized Losses	(13,563)	(30,418)
Total Unrealized Losses	$ (27,395)	$ (48,600)
Number of Securities
Less than 12 Months Number of Securities | item	98	135
12 Months or More Number of Securities | item	43	70
Total Number of Securities | item	141	205
20 Year Mortgage
Fair Value
Fair value - Less than 12 months	$ 3,000	$ 142,749
12 Months or More Fair Value	0	0
Total Fair Value	3,000	142,749
Unrealized Losses
Less than 12 Months Unrealized Losses	(12)	(1,935)
12 Months or More Unrealized Losses	0	0
Total Unrealized Losses	$ (12)	$ (1,935)
Number of Securities
Less than 12 Months Number of Securities | item	3	47
12 Months or More Number of Securities | item	0	0
Total Number of Securities | item	3	47
30 Year Mortgage
Fair Value
Fair value - Less than 12 months	$ 44,072	$ 432,949
12 Months or More Fair Value	1,632	22,586
Total Fair Value	45,704	455,535
Unrealized Losses
Less than 12 Months Unrealized Losses	(291)	(11,264)
12 Months or More Unrealized Losses	(135)	(945)
Total Unrealized Losses	$ (426)	$ (12,209)
Number of Securities
Less than 12 Months Number of Securities | item	7	54
12 Months or More Number of Securities | item	5	13
Total Number of Securities | item	12	67
40 Years Mortgage
Fair Value
Fair value - Less than 12 months	$ 283,187
12 Months or More Fair Value	0
Total Fair Value	283,187
Unrealized Losses
Less than 12 Months Unrealized Losses	(1,876)
12 Months or More Unrealized Losses	0
Total Unrealized Losses	$ (1,876)
Number of Securities
Less than 12 Months Number of Securities | item	1
12 Months or More Number of Securities | item	0
Total Number of Securities | item	1
Agency RMBS Interest Only Strips
Fair Value
Fair value - Less than 12 months	$ 3,095	$ 6,105
12 Months or More Fair Value	1,703	1,630
Total Fair Value	4,798	7,735
Unrealized Losses
Less than 12 Months Unrealized Losses	(142)	(227)
12 Months or More Unrealized Losses	(49)	(94)
Total Unrealized Losses	$ (191)	$ (321)
Number of Securities
Less than 12 Months Number of Securities | item	6	6
12 Months or More Number of Securities | item	3	2
Total Number of Securities | item	9	8
Agency CMBS
Fair Value
Fair value - Less than 12 months	$ 955,559	$ 145,791
Total Fair Value	955,559	145,791
Unrealized Losses
Less than 12 Months Unrealized Losses	(9,394)	(260)
Total Unrealized Losses	$ (9,394)	$ (260)
Number of Securities
Less than 12 Months Number of Securities | item	57	7
Total Number of Securities | item	57	7
Subtotal Agency MBS
Fair Value
Fair value - Less than 12 months	$ 1,288,913	$ 727,594
12 Months or More Fair Value	3,335	24,216
Total Fair Value	1,292,248	751,810
Unrealized Losses
Less than 12 Months Unrealized Losses	(11,715)	(13,686)
12 Months or More Unrealized Losses	(184)	(1,039)
Total Unrealized Losses	$ (11,899)	$ (14,725)
Number of Securities
Less than 12 Months Number of Securities | item	74	114
12 Months or More Number of Securities | item	8	15
Total Number of Securities | item	82	129
Non-Agency RMBS
Fair Value
Fair value - Less than 12 months	$ 28,508	$ 11,628
12 Months or More Fair Value	0	33,034
Total Fair Value	28,508	44,662
Unrealized Losses
Less than 12 Months Unrealized Losses	(161)	(50)
12 Months or More Unrealized Losses	0	(2,185)
Total Unrealized Losses	$ (161)	$ (2,235)
Number of Securities
Less than 12 Months Number of Securities | item	3	3
12 Months or More Number of Securities | item	0	6
Total Number of Securities | item	3	9
Non-Agency RMBS Interest-Only Strips
Fair Value
Fair value - Less than 12 months	$ 8,722
12 Months or More Fair Value	0
Total Fair Value	8,722
Unrealized Losses
Less than 12 Months Unrealized Losses	(16)
12 Months or More Unrealized Losses	0
Total Unrealized Losses	$ (16)
Number of Securities
Less than 12 Months Number of Securities | item	3
12 Months or More Number of Securities | item	0
Total Number of Securities | item	3
Non-Agency CMBS
Fair Value
Fair value - Less than 12 months	$ 69,661	$ 59,529
12 Months or More Fair Value	119,729	208,288
Total Fair Value	189,390	267,817
Unrealized Losses
Less than 12 Months Unrealized Losses	(1,753)	(4,031)
12 Months or More Unrealized Losses	(13,379)	(26,015)
Total Unrealized Losses	$ (15,132)	$ (30,046)
Number of Securities
Less than 12 Months Number of Securities | item	15	17
12 Months or More Number of Securities | item	35	47
Total Number of Securities | item	50	64
Subtotal Non-Agency MBS
Fair Value
Fair value - Less than 12 months	$ 106,891	$ 71,157
12 Months or More Fair Value	119,729	241,322
Total Fair Value	226,620	312,479
Unrealized Losses
Less than 12 Months Unrealized Losses	(1,930)	(4,081)
12 Months or More Unrealized Losses	(13,379)	(28,200)
Total Unrealized Losses	$ (15,309)	$ (32,281)
Number of Securities
Less than 12 Months Number of Securities | item	21	20
12 Months or More Number of Securities | item	35	53
Total Number of Securities | item	56	73
Other securities
Fair Value
Fair value - Less than 12 months	$ 23,800	$ 7,966
12 Months or More Fair Value	0	23,390
Total Fair Value	23,800	31,356
Unrealized Losses
Less than 12 Months Unrealized Losses	(187)	(415)
12 Months or More Unrealized Losses	0	(1,179)
Total Unrealized Losses	$ (187)	$ (1,594)
Number of Securities
Less than 12 Months Number of Securities | item	3	1
12 Months or More Number of Securities | item	0	2
Total Number of Securities | item	3	3