Information About Company's Adjustable Rate Agency Securities That will Reprice or Amortize Based on Contractual terms (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]
Fair Value	$ 23,919,251	$ 17,741,873
Fixed Rate Securities
Mortgage Loans on Real Estate [Line Items]
Fair Value	1,508,589	987,939
% of Total	100.00%	100.00%
Weighted Average Coupon	2.62%	3.05%
Fixed Rate Securities | 155 - 167 Months
Mortgage Loans on Real Estate [Line Items]
Fair Value	102,158
% of Total	6.80%
Weighted Average Coupon	3.00%
Fixed Rate Securities | 168 - 180 Months
Mortgage Loans on Real Estate [Line Items]
Fair Value	1,406,431	987,939
% of Total	93.20%	100.00%
Weighted Average Coupon	2.60%	3.05%
Fixed Rate Securities | Minimum | 155 - 167 Months
Mortgage Loans on Real Estate [Line Items]
Wtd Average Months to Maturity	155
Fixed Rate Securities | Minimum | 168 - 180 Months
Mortgage Loans on Real Estate [Line Items]
Wtd Average Months to Maturity	168
Fixed Rate Securities | Maximum | 155 - 167 Months
Mortgage Loans on Real Estate [Line Items]
Wtd Average Months to Maturity	167
Fixed Rate Securities | Maximum | 168 - 180 Months
Mortgage Loans on Real Estate [Line Items]
Wtd Average Months to Maturity	180
Adjustable Rate Securities
Mortgage Loans on Real Estate [Line Items]
Fair Value	22,410,662	16,753,934
% of Total	100.00%	100.00%
Weighted Average Coupon	2.95%	3.49%
Adjustable Rate Securities | 0 - 12 Months
Mortgage Loans on Real Estate [Line Items]
Fair Value	914,832	691,226
% of Total	4.10%	4.00%
Weighted Average Coupon	4.03%	4.56%
Adjustable Rate Securities | 13 - 24 Months
Mortgage Loans on Real Estate [Line Items]
Fair Value	966,854	769,239
% of Total	4.30%	4.60%
Weighted Average Coupon	3.88%	4.93%
Adjustable Rate Securities | 25 - 36 Months
Mortgage Loans on Real Estate [Line Items]
Fair Value	2,274,763	992,880
% of Total	10.20%	5.90%
Weighted Average Coupon	3.60%	3.93%
Adjustable Rate Securities | 37 - 48 Months
Mortgage Loans on Real Estate [Line Items]
Fair Value	3,207,245	3,669,359
% of Total	14.30%	21.90%
Weighted Average Coupon	3.06%	3.65%
Adjustable Rate Securities | 49 - 60 Months
Mortgage Loans on Real Estate [Line Items]
Fair Value	3,616,891	4,439,416
% of Total	16.10%	26.50%
Weighted Average Coupon	2.76%	3.06%
Adjustable Rate Securities | 61 - 72 Months
Mortgage Loans on Real Estate [Line Items]
Fair Value	3,027,295	2,071,610
% of Total	13.50%	12.40%
Weighted Average Coupon	3.26%	3.41%
Adjustable Rate Securities | 73 - 84 Months
Mortgage Loans on Real Estate [Line Items]
Fair Value	7,607,359	3,965,045
% of Total	33.90%	23.70%
Weighted Average Coupon	2.44%	3.28%
Adjustable Rate Securities | 85 - 96 Months
Mortgage Loans on Real Estate [Line Items]
Fair Value	133,487	9,866
% of Total	0.60%	0.10%
Weighted Average Coupon	3.08%	4.33%
Adjustable Rate Securities | 97 - 108 Months
Mortgage Loans on Real Estate [Line Items]
Fair Value		145,293
% of Total		0.90%
Weighted Average Coupon		3.93%
Adjustable Rate Securities | 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 Securities | 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 Securities | Minimum | 0 - 12 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	0
Adjustable Rate Securities | Minimum | 13 - 24 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	13
Adjustable Rate Securities | Minimum | 25 - 36 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	25
Adjustable Rate Securities | Minimum | 37 - 48 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	37
Adjustable Rate Securities | Minimum | 49 - 60 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	49
Adjustable Rate Securities | Minimum | 61 - 72 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	61
Adjustable Rate Securities | Minimum | 73 - 84 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	73
Adjustable Rate Securities | Minimum | 85 - 96 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	85
Adjustable Rate Securities | Minimum | 97 - 108 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	97
Adjustable Rate Securities | Minimum | 109 - 120 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	109
Adjustable Rate Securities | Minimum | 121 - 140 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	121
Adjustable Rate Securities | Maximum | 0 - 12 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	12
Adjustable Rate Securities | Maximum | 13 - 24 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	24
Adjustable Rate Securities | Maximum | 25 - 36 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	36
Adjustable Rate Securities | Maximum | 37 - 48 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	48
Adjustable Rate Securities | Maximum | 49 - 60 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	60
Adjustable Rate Securities | Maximum | 61 - 72 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	72
Adjustable Rate Securities | Maximum | 73 - 84 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	84
Adjustable Rate Securities | Maximum | 85 - 96 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	96
Adjustable Rate Securities | Maximum | 97 - 108 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	108
Adjustable Rate Securities | Maximum | 109 - 120 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	120
Adjustable Rate Securities | Maximum | 121 - 140 Months
Mortgage Loans on Real Estate [Line Items]
Months to Coupon Reset or Contractual Payment	140