Information About Company's Adjustable Rate Agency Securities That will Reprice or Amortize Based on Contractual terms (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value	$ 14,302,230	$ 17,587,010
Minimum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Coupon	2.63%	2.75%
Maximum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Coupon	4.00%	3.75%
Adjustable Rate Residential Mortgage
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value	$ 13,414,307	$ 16,310,376
% of Total	100.00%	100.00%
Weighted Average Coupon	2.67%	2.75%
Adjustable Rate Residential Mortgage | 0 - 12 Months
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value	$ 2,377,014	$ 2,272,988
% of Total	17.70%	14.00%
Weighted Average Coupon	2.84%	3.02%
Adjustable Rate Residential Mortgage | 0 - 12 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	0 months
Adjustable Rate Residential Mortgage | 0 - 12 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	12 months
Adjustable Rate Residential Mortgage | 13 - 24 Months
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value	$ 1,467,597	$ 1,799,368
% of Total	10.90%	11.00%
Weighted Average Coupon	2.69%	2.97%
Adjustable Rate Residential Mortgage | 13 - 24 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	13 months
Adjustable Rate Residential Mortgage | 13 - 24 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	24 months
Adjustable Rate Residential Mortgage | 25 - 36 Months
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value	$ 1,692,591	$ 2,026,261
% of Total	12.60%	12.40%
Weighted Average Coupon	2.87%	2.73%
Adjustable Rate Residential Mortgage | 25 - 36 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	25 months
Adjustable Rate Residential Mortgage | 25 - 36 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	36 months
Adjustable Rate Residential Mortgage | 37 - 48 Months
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value	$ 4,805,462	$ 2,268,665
% of Total	35.80%	13.90%
Weighted Average Coupon	2.50%	2.89%
Adjustable Rate Residential Mortgage | 37 - 48 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	37 months
Adjustable Rate Residential Mortgage | 37 - 48 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	48 months
Adjustable Rate Residential Mortgage | 49 - 60 Months
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value	$ 1,338,716	$ 5,855,524
% of Total	10.00%	35.90%
Weighted Average Coupon	2.43%	2.52%
Adjustable Rate Residential Mortgage | 49 - 60 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	49 months
Adjustable Rate Residential Mortgage | 49 - 60 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	60 months
Adjustable Rate Residential Mortgage | 61 - 72 Months
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value	$ 1,015,398	$ 880,039
% of Total	7.60%	5.40%
Weighted Average Coupon	2.95%	2.52%
Adjustable Rate Residential Mortgage | 61 - 72 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	61 months
Adjustable Rate Residential Mortgage | 61 - 72 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	72 months
Adjustable Rate Residential Mortgage | 73 - 84 Months
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value	$ 684,191	$ 1,207,531
% of Total	5.10%	7.40%
Weighted Average Coupon	2.73%	2.98%
Adjustable Rate Residential Mortgage | 73 - 84 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	73 months
Adjustable Rate Residential Mortgage | 73 - 84 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	84 months
Adjustable Rate Residential Mortgage | 109 - 120 Months
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value	$ 33,338
% of Total	0.30%
Weighted Average Coupon	2.85%
Adjustable Rate Residential Mortgage | 109 - 120 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	109 months
Adjustable Rate Residential Mortgage | 109 - 120 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	120 months
Fixed Rate Residential Mortgage
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value	$ 997,310	$ 1,276,634
% of Total	100.00%	100.00%
Weighted Average Coupon	3.50%	3.47%
Fixed Rate Residential Mortgage | 121-132 Months
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value		$ 73,814
% of Total		5.80%
Weighted Average Coupon		3.50%
Fixed Rate Residential Mortgage | 121-132 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	121 months
Fixed Rate Residential Mortgage | 121-132 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	132 months
Fixed Rate Residential Mortgage | 133-144 Months
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value		$ 589,051
% of Total		46.10%
Weighted Average Coupon		3.50%
Fixed Rate Residential Mortgage | 133-144 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	133 months
Fixed Rate Residential Mortgage | 133-144 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	144 months
Fixed Rate Residential Mortgage | 145-156 Months
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value		$ 613,769
% of Total		48.10%
Weighted Average Coupon		3.43%
Fixed Rate Residential Mortgage | 145-156 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	145 months
Fixed Rate Residential Mortgage | 145-156 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	156 months
Fixed Rate Residential Mortgage | 349-360 Months
Mortgage Loans On Real Estate [Line Items]
MBS, Fair Value	$ 997,310
% of Total	100.00%
Weighted Average Coupon	3.50%
Fixed Rate Residential Mortgage | 349-360 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	349 months
Fixed Rate Residential Mortgage | 349-360 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	360 months