Repurchase Agreements - Remaining Maturities, Collateral Types and Weighted Average Rate (Detail) - USD ($) $ in Thousands		Jun. 30, 2017		Dec. 31, 2016
Repurchase Agreements:
Repurchase agreements		$ 62,497,400	[1]	$ 65,215,810	[2]
Weighted average rate		1.38%		1.07%
Credit Risk Transfer Securities
Repurchase Agreements:
Repurchase agreements		$ 392,483		$ 453,025
Non Agency MBS
Repurchase Agreements:
Repurchase agreements		704,794		720,217
Commercial Loan
Repurchase Agreements:
Repurchase agreements		394,608		334,867
Agency Mortgage-backed Securities
Repurchase Agreements:
Repurchase agreements		60,980,074		63,707,701
Commercial Mortgage-backed Securities
Repurchase Agreements:
Repurchase agreements		25,441
2 to 29 days
Repurchase Agreements:
Repurchase agreements		$ 26,626,575		$ 30,394,043
Weighted average rate		1.40%		0.87%
2 to 29 days | Credit Risk Transfer Securities
Repurchase Agreements:
Repurchase agreements		$ 306,812		$ 358,972
2 to 29 days | Non Agency MBS
Repurchase Agreements:
Repurchase agreements		434,128		377,366
2 to 29 days | Agency Mortgage-backed Securities
Repurchase Agreements:
Repurchase agreements		25,866,226		29,657,705
2 to 29 days | Commercial Mortgage-backed Securities
Repurchase Agreements:
Repurchase agreements		19,409
30 to 59 days
Repurchase Agreements:
Repurchase agreements		$ 8,123,046		$ 11,694,799
Weighted average rate		1.33%		1.10%
30 to 59 days | Credit Risk Transfer Securities
Repurchase Agreements:
Repurchase agreements		$ 49,657		$ 80,139
30 to 59 days | Non Agency MBS
Repurchase Agreements:
Repurchase agreements		136,613		241,360
30 to 59 days | Agency Mortgage-backed Securities
Repurchase Agreements:
Repurchase agreements		7,930,744		11,373,300
30 to 59 days | Commercial Mortgage-backed Securities
Repurchase Agreements:
Repurchase agreements		6,032
60 to 89 days
Repurchase Agreements:
Repurchase agreements		$ 9,276,299		$ 7,082,232
Weighted average rate		1.33%		1.14%
60 to 89 days | Credit Risk Transfer Securities
Repurchase Agreements:
Repurchase agreements		$ 36,014		$ 13,914
60 to 89 days | Non Agency MBS
Repurchase Agreements:
Repurchase agreements		129,094		101,491
60 to 89 days | Agency Mortgage-backed Securities
Repurchase Agreements:
Repurchase agreements		9,111,191		6,966,827
90 to 119 days
Repurchase Agreements:
Repurchase agreements		$ 3,100,967		$ 2,063,561
Weighted average rate		1.20%		0.89%
90 to 119 days | Non Agency MBS
Repurchase Agreements:
Repurchase agreements		$ 4,959
90 to 119 days | Agency Mortgage-backed Securities
Repurchase Agreements:
Repurchase agreements		3,096,008		$ 2,063,561
Over 120 days
Repurchase Agreements:
Repurchase agreements	[3]	$ 15,370,513		$ 13,981,175
Weighted average rate	[3]	1.45%		1.47%
Over 120 days | Commercial Loan
Repurchase Agreements:
Repurchase agreements	[3]	$ 394,608		$ 334,867
Over 120 days | Agency Mortgage-backed Securities
Repurchase Agreements:
Repurchase agreements	[3]	$ 14,975,905		$ 13,646,308

[1] As a result of a change to a clearing organization's rulebook effective January 3, 2017, beginning with the first quarter 2017 and in subsequent periods the Company is presenting the fair value of centrally cleared interest rate swaps net of variation margin pledged under such transactions. The variation margin was previously reported under cash and cash equivalents and is currently reported as a reduction to interest rate swaps, at fair value. Balances reported prior to the effective date balances will not be adjusted.

[2] Derived from the audited consolidated financial statements at December 31, 2016.

[3] Approximately 5% and 7% of the total repurchase agreements had a remaining maturity over 1 year as of March 31, 2017 and December 31, 2016, respectively.