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, 2013	Dec. 31, 2012
Mortgage Loans On Real Estate [Line Items]
Fair Value	$ 17,642,532	$ 23,919,251
Adjustable Rate Residential Mortgage
Mortgage Loans On Real Estate [Line Items]
Fair Value	16,401,789	22,410,662
% of Total	100.00%	100.00%
Weighted Average Coupon	2.80%	2.95%
Adjustable Rate Residential Mortgage | 0 - 12 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value	1,236,878	914,832
% of Total	7.50%	4.10%
Weighted Average Coupon	3.15%	4.03%
Adjustable Rate Residential Mortgage | 0 - 12 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	0
Adjustable Rate Residential Mortgage | 0 - 12 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	12
Adjustable Rate Residential Mortgage | 13 - 24 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value	1,614,081	966,854
% of Total	9.80%	4.30%
Weighted Average Coupon	3.49%	3.88%
Adjustable Rate Residential Mortgage | 13 - 24 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	13
Adjustable Rate Residential Mortgage | 13 - 24 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	24
Adjustable Rate Residential Mortgage | 25 - 36 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value	2,321,985	2,274,763
% of Total	14.20%	10.20%
Weighted Average Coupon	2.99%	3.60%
Adjustable Rate Residential Mortgage | 25 - 36 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	25
Adjustable Rate Residential Mortgage | 25 - 36 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	36
Adjustable Rate Residential Mortgage | 37 - 48 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value	2,524,939	3,207,245
% of Total	15.40%	14.30%
Weighted Average Coupon	2.73%	3.06%
Adjustable Rate Residential Mortgage | 37 - 48 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	37
Adjustable Rate Residential Mortgage | 37 - 48 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	48
Adjustable Rate Residential Mortgage | 49 - 60 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value	2,483,318	3,616,891
% of Total	15.10%	16.10%
Weighted Average Coupon	2.94%	2.76%
Adjustable Rate Residential Mortgage | 49 - 60 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	49
Adjustable Rate Residential Mortgage | 49 - 60 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	60
Adjustable Rate Residential Mortgage | 61 - 72 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value	4,750,098	3,027,295
% of Total	29.00%	13.50%
Weighted Average Coupon	2.46%	3.26%
Adjustable Rate Residential Mortgage | 61 - 72 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	61
Adjustable Rate Residential Mortgage | 61 - 72 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	72
Adjustable Rate Residential Mortgage | 73 - 84 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value	1,470,490	7,607,359
% of Total	9.00%	33.90%
Weighted Average Coupon	2.44%	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
Adjustable Rate Residential Mortgage | 73 - 84 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	84
Adjustable Rate Residential Mortgage | 85 - 96 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value		133,487
% of Total		0.60%
Weighted Average Coupon		3.08%
Adjustable Rate Residential Mortgage | 85 - 96 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	85
Adjustable Rate Residential Mortgage | 85 - 96 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	96
Adjustable Rate Residential Mortgage | 97 - 108 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value
% of Total
Weighted Average Coupon
Adjustable Rate Residential Mortgage | 97 - 108 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	97
Adjustable Rate Residential Mortgage | 97 - 108 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	108
Adjustable Rate Residential Mortgage | 109 - 120 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value		641,013
% of Total		2.90%
Weighted Average Coupon		2.81%
Adjustable Rate Residential Mortgage | 109 - 120 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	109
Adjustable Rate Residential Mortgage | 109 - 120 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	120
Adjustable Rate Residential Mortgage | 121 - 140 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value		20,923
% of Total		0.10%
Weighted Average Coupon		2.69%
Adjustable Rate Residential Mortgage | 121 - 140 Months | Minimum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	121
Adjustable Rate Residential Mortgage | 121 - 140 Months | Maximum
Mortgage Loans On Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	140
Fixed Rate Residential Mortgage
Mortgage Loans On Real Estate [Line Items]
Fair Value	1,240,743	1,508,589
% of Total	100.00%	100.00%
Weighted Average Coupon	3.46%	2.62%
Fixed Rate Residential Mortgage | 133-144 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value	75,693
% of Total	6.10%
Weighted Average Coupon	3.50%
Fixed Rate Residential Mortgage | 145-156 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value	477,544	102,158
% of Total	38.50%	6.80%
Weighted Average Coupon	3.50%	3.00%
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	3.00%
Fixed Rate Residential Mortgage | 169-180 Months
Mortgage Loans On Real Estate [Line Items]
Fair Value	$ 592,268	$ 1,406,431
% of Total	47.70%	93.20%
Weighted Average Coupon	3.50%	2.60%
Fixed Rate Residential Mortgage | Minimum | 133-144 Months
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	133
Fixed Rate Residential Mortgage | Minimum | 145-156 Months
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	145
Fixed Rate Residential Mortgage | Minimum | 157-168 Months
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	157
Fixed Rate Residential Mortgage | Minimum | 169-180 Months
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	169
Fixed Rate Residential Mortgage | Maximum | 133-144 Months
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	144
Fixed Rate Residential Mortgage | Maximum | 145-156 Months
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	156
Fixed Rate Residential Mortgage | Maximum | 157-168 Months
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	168
Fixed Rate Residential Mortgage | Maximum | 169-180 Months
Mortgage Loans On Real Estate [Line Items]
Weighted Average Months to Maturity	180