Information About Company's Adjustable Rate Agency Securities That will Reprice or Amortize Based on Contractual terms (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Mortgage Loans On Real Estate [Line Items]
Fair Value	$ 17,587,010	$ 17,642,532
Minimum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Coupon	2.75%
Maximum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Coupon	3.75%
Adjustable Rate Residential Mortgage
Mortgage Loans On Real Estate [Line Items]
Fair Value	16,310,376	16,401,789
% of Total	100.00%	100.00%
Weighted Average Coupon	2.75%	2.80%
Adjustable Rate Residential Mortgage | 0 - 12 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value	2,272,988	1,236,878
% of Total	14.00%	7.50%
Weighted Average Coupon	3.02%	3.15%
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]
Fair Value	1,799,368	1,614,081
% of Total	11.00%	9.80%
Weighted Average Coupon	2.97%	3.49%
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]
Fair Value	2,026,261	2,321,985
% of Total	12.40%	14.20%
Weighted Average Coupon	2.73%	2.99%
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]
Fair Value	2,268,665	2,524,939
% of Total	13.90%	15.40%
Weighted Average Coupon	2.89%	2.73%
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]
Fair Value	5,855,524	2,483,318
% of Total	35.90%	15.10%
Weighted Average Coupon	2.52%	2.94%
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]
Fair Value	880,039	4,750,098
% of Total	5.40%	29.00%
Weighted Average Coupon	2.52%	2.46%
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]
Fair Value	1,207,531	1,470,490
% of Total	7.40%	9.00%
Weighted Average Coupon	2.98%	2.44%
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
Fixed Rate Residential Mortgage
Mortgage Loans On Real Estate [Line Items]
Fair Value	1,276,634	1,240,743
% of Total	100.00%	100.00%
Weighted Average Coupon	3.47%	3.46%
Fixed Rate Residential Mortgage | 121-132 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value	73,814
% of Total	5.80%
Weighted Average Coupon	3.50%
Fixed Rate Residential Mortgage | 133-144 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value	589,051	75,693
% of Total	46.10%	6.10%
Weighted Average Coupon	3.50%	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]
Fair Value	613,769	477,544
% of Total	48.10%	38.50%
Weighted Average Coupon	3.43%	3.50%
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 | 157-168 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value		95,238
% of Total		7.70%
Weighted Average Coupon	0.00%	3.00%
Fixed Rate Residential Mortgage | 157-168 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	157 months
Fixed Rate Residential Mortgage | 157-168 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	168 months
Fixed Rate Residential Mortgage | 169-180 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value		$ 592,268
% of Total		47.70%
Weighted Average Coupon	0.00%	3.50%
Fixed Rate Residential Mortgage | 169-180 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	169 months
Fixed Rate Residential Mortgage | 169-180 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	180 months