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Mortgage-Backed Securities and other securities (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Mortgage-backed securities and other securities    
Mortgage-Backed Securities and other securities    
Principal Balance $ 3,371,419 $ 2,585,481
Unamortized Premium (Discount), net (16,548) 8,671
Discount Designated as Credit Reserve and OTTI (72,915) (130,484)
Amortized Cost 3,328,334 2,561,552
Unrealized Gain 41,708 36,248
Unrealized Loss (27,395) (48,600)
Estimated Fair Value $ 3,358,823 $ 2,576,517
Net Weighted Average Coupon (as a percent) 3.10% 3.90%
Weighted average expected remaining term to the expected maturity of investment portfolio 8 years 7 months 6 days 7 years 1 month 6 days
20 Year Mortgage    
Mortgage-Backed Securities and other securities    
Principal Balance $ 50,825 $ 470,975
Unamortized Premium (Discount), net 2,378 25,741
Amortized Cost 53,203 496,716
Unrealized Gain 592 3,689
Unrealized Loss (12) (1,935)
Estimated Fair Value $ 53,783 $ 498,470
Net Weighted Average Coupon (as a percent) 4.00% 3.90%
30 Year Mortgage    
Mortgage-Backed Securities and other securities    
Principal Balance $ 224,041 $ 878,599
Unamortized Premium (Discount), net 15,710 63,608
Amortized Cost 239,751 942,207
Unrealized Gain 2,317 5,209
Unrealized Loss (426) (12,209)
Estimated Fair Value $ 241,642 $ 935,207
Net Weighted Average Coupon (as a percent) 4.40% 4.10%
40 Years Mortgage    
Mortgage-Backed Securities and other securities    
Principal Balance $ 366,178  
Unamortized Premium (Discount), net 10,788  
Amortized Cost 376,966  
Unrealized Gain 1,662  
Unrealized Loss (1,876)  
Estimated Fair Value $ 376,752  
Net Weighted Average Coupon (as a percent) 3.50%  
Agency RMBS Interest Only Strips    
Mortgage-Backed Securities and other securities    
Amortized Cost $ 14,750 $ 18,810
Unrealized Gain 878 1,301
Unrealized Loss (191) (321)
Estimated Fair Value $ 15,437 $ 19,790
Net Weighted Average Coupon (as a percent) 2.90% 3.00%
Notional balance $ 165,500 $ 201,600
Agency RMBS Interest-Only Strips accounted for as derivatives, included in MBS    
Mortgage-Backed Securities and other securities    
Estimated Fair Value $ 10,419 $ 16,503
Net Weighted Average Coupon (as a percent) 2.90% 3.20%
Notional balance $ 122,000 $ 188,100
Agency CMBS    
Mortgage-Backed Securities and other securities    
Principal Balance 2,145,139 377,286
Unamortized Premium (Discount), net 2,142 (15,383)
Amortized Cost 2,147,281 361,903
Unrealized Gain 16,913 2,021
Unrealized Loss (9,394) (260)
Estimated Fair Value $ 2,154,800 $ 363,664
Net Weighted Average Coupon (as a percent) 2.90% 2.60%
Agency CMBS Interest-Only Strips    
Mortgage-Backed Securities and other securities    
Amortized Cost $ 0 $ 210
Unrealized Gain 10 21
Estimated Fair Value $ 10 $ 231
Net Weighted Average Coupon (as a percent) 3.20% 4.30%
Notional balance $ 3,300 $ 32,800
Agency CMBS Interest-Only Strips accounted for as derivatives, included in MBS    
Mortgage-Backed Securities and other securities    
Estimated Fair Value $ 5,757 $ 7,729
Net Weighted Average Coupon (as a percent) 0.50% 0.60%
Notional balance $ 192,500 $ 221,800
Subtotal Agency MBS    
Mortgage-Backed Securities and other securities    
Principal Balance 2,786,183 1,726,860
Unamortized Premium (Discount), net 31,018 73,966
Amortized Cost 2,831,951 1,819,846
Unrealized Gain 22,372 12,241
Unrealized Loss (11,899) (14,725)
Estimated Fair Value $ 2,858,600 $ 1,841,594
Net Weighted Average Coupon (as a percent) 3.00% 3.30%
Non-Agency RMBS    
Mortgage-Backed Securities and other securities    
Principal Balance $ 119,748 $ 340,759
Unamortized Premium (Discount), net 5,263 (294)
Discount Designated as Credit Reserve and OTTI (39,491) (108,399)
Amortized Cost 85,520 232,066
Unrealized Gain 5,473 11,210
Unrealized Loss (161) (2,235)
Estimated Fair Value $ 90,832 $ 241,041
Net Weighted Average Coupon (as a percent) 3.80% 4.50%
Non-Agency RMBS Interest-Only Strips    
Mortgage-Backed Securities and other securities    
Amortized Cost $ 8,738 $ 55,754
Unrealized Gain 0 8,362
Unrealized Loss (16)  
Estimated Fair Value $ 8,722 $ 64,116
Net Weighted Average Coupon (as a percent) 0.90% 5.60%
Notional balance $ 278,400 $ 278,400
Non-Agency RMBS Interest Only Strips Accounted For As Derivatives Issued By Private Enterprises    
Mortgage-Backed Securities and other securities    
Estimated Fair Value   $ 3,085
Net Weighted Average Coupon (as a percent)   4.60%
Notional balance   $ 20,700
Non-Agency CMBS    
Mortgage-Backed Securities and other securities    
Principal Balance 379,183 473,024
Unamortized Premium (Discount), net (59,129) (69,436)
Discount Designated as Credit Reserve and OTTI (28,020) (17,787)
Amortized Cost 292,034 385,801
Unrealized Gain 1,702 3,164
Unrealized Loss (15,132) (30,046)
Estimated Fair Value $ 278,604 $ 358,919
Net Weighted Average Coupon (as a percent) 4.80% 5.00%
Subtotal Non-Agency MBS    
Mortgage-Backed Securities and other securities    
Principal Balance $ 498,931 $ 813,783
Unamortized Premium (Discount), net (53,866) (69,730)
Discount Designated as Credit Reserve and OTTI (67,511) (126,186)
Amortized Cost 386,292 673,621
Unrealized Gain 7,175 22,736
Unrealized Loss (15,309) (32,281)
Estimated Fair Value $ 378,158 $ 667,161
Net Weighted Average Coupon (as a percent) 3.30% 5.00%
Other securities    
Mortgage-Backed Securities and other securities    
Principal Balance $ 86,305 $ 44,838
Unamortized Premium (Discount), net 6,300 4,435
Discount Designated as Credit Reserve and OTTI (5,404) (4,298)
Amortized Cost 110,091 68,085
Unrealized Gain 12,161 1,271
Unrealized Loss (187) (1,594)
Estimated Fair Value $ 122,065 $ 67,762
Net Weighted Average Coupon (as a percent) 7.80% 8.20%
Residual interests in asset-backed securities    
Mortgage-Backed Securities and other securities    
Principal Balance   $ 0
Amortized Cost $ 22,900 $ 23,100