Repurchase Agreements - Remaining Maturities, Collateral Types and Weighted Average Rate (Detail) - USD ($) $ in Thousands		Sep. 30, 2015		Dec. 31, 2014
Repurchase Agreements:
Repurchase Agreements		$ 56,449,364		$ 71,361,926	[1]
Weighted average rate		0.78%		0.61%
Debentures
Repurchase Agreements:
Repurchase Agreements		$ 94,950		$ 1,314,248
Credit Risk Transfer Securities
Repurchase Agreements:
Repurchase Agreements		78,194
Non Agency MBS
Repurchase Agreements:
Repurchase Agreements		187,108
Commercial Loan [Member]
Repurchase Agreements:
Repurchase Agreements		150,628
Agency Mortgage Backed Securities
Repurchase Agreements:
Repurchase Agreements		55,938,484		$ 70,047,678
1 day
Repurchase Agreements:
Repurchase Agreements		$ 8,050,000
Weighted average rate		0.57%		0.00%
1 day | Agency Mortgage Backed Securities
Repurchase Agreements:
Repurchase Agreements		$ 8,050,000
2 to 29 days
Repurchase Agreements:
Repurchase Agreements		$ 11,830,862		$ 28,354,167
Weighted average rate		0.45%		0.35%
2 to 29 days | Debentures
Repurchase Agreements:
Repurchase Agreements		$ 94,950		$ 749,535
2 to 29 days | Credit Risk Transfer Securities
Repurchase Agreements:
Repurchase Agreements		17,788
2 to 29 days | Non Agency MBS
Repurchase Agreements:
Repurchase Agreements		77,236
2 to 29 days | Agency Mortgage Backed Securities
Repurchase Agreements:
Repurchase Agreements		11,640,888		27,604,632
30 to 59 days
Repurchase Agreements:
Repurchase Agreements		$ 4,846,173		$ 17,336,469
Weighted average rate		0.52%		0.43%
30 to 59 days | Debentures
Repurchase Agreements:
Repurchase Agreements				$ 186,682
30 to 59 days | Credit Risk Transfer Securities
Repurchase Agreements:
Repurchase Agreements		$ 60,406
30 to 59 days | Non Agency MBS
Repurchase Agreements:
Repurchase Agreements		63,852
30 to 59 days | Agency Mortgage Backed Securities
Repurchase Agreements:
Repurchase Agreements		4,721,915		17,149,787
60 to 89 days
Repurchase Agreements:
Repurchase Agreements		$ 8,840,129		$ 4,040,677
Weighted average rate		0.57%		0.38%
60 to 89 days | Debentures
Repurchase Agreements:
Repurchase Agreements				$ 378,031
60 to 89 days | Non Agency MBS
Repurchase Agreements:
Repurchase Agreements		$ 46,020
60 to 89 days | Agency Mortgage Backed Securities
Repurchase Agreements:
Repurchase Agreements		8,794,109		3,662,646
90 to 119 days
Repurchase Agreements:
Repurchase Agreements		$ 3,957,380		$ 2,945,495
Weighted average rate		0.52%		0.50%
90 to 119 days | Agency Mortgage Backed Securities
Repurchase Agreements:
Repurchase Agreements		$ 3,957,380		$ 2,945,495
Over 120 days
Repurchase Agreements:
Repurchase Agreements	[2]	$ 18,924,820		$ 18,685,118
Weighted average rate		1.29%	[2]	1.24%
Over 120 days | Commercial Loan [Member]
Repurchase Agreements:
Repurchase Agreements	[2]	$ 150,628
Over 120 days | Agency Mortgage Backed Securities
Repurchase Agreements:
Repurchase Agreements	[2]	$ 18,774,192		$ 18,685,118

[1] Derived from the audited consolidated financial statements at December 31, 2014.

[2] Approximately 14% and 15% of the total repurchase agreements had a remaining maturity over 1 year as of September 30, 2015 and December 31, 2014, respectively.