Repurchase Agreements - Remaining Maturities, Collateral Types and Weighted Average Rate (Detail) - USD ($) $ in Thousands		Jun. 30, 2015		Dec. 31, 2014
Repurchase Agreements:
Repurchase Agreements		$ 57,459,552		$ 71,361,926	[1]
Weighted average rate		0.76%		0.61%
Agency Mortgage Backed Securities And Debentures [Member]
Repurchase Agreements:
Repurchase Agreements		$ 57,337,552		$ 71,361,926
Commercial Loan [Member]
Repurchase Agreements:
Repurchase Agreements		122,000
1 day
Repurchase Agreements:
Repurchase Agreements		$ 7,150,000
Weighted average rate		0.82%		0.00%
1 day | Agency Mortgage Backed Securities And Debentures [Member]
Repurchase Agreements:
Repurchase Agreements		$ 7,150,000
2 to 29 days
Repurchase Agreements:
Repurchase Agreements		$ 16,013,749
Weighted average rate		0.43%		0.35%
2 to 29 days | Agency Mortgage Backed Securities And Debentures [Member]
Repurchase Agreements:
Repurchase Agreements		$ 16,013,749		$ 28,354,167
30 to 59 days
Repurchase Agreements:
Repurchase Agreements		$ 8,157,729
Weighted average rate		0.52%		0.43%
30 to 59 days | Agency Mortgage Backed Securities And Debentures [Member]
Repurchase Agreements:
Repurchase Agreements		$ 8,157,729		$ 17,336,469
60 to 89 days
Repurchase Agreements:
Repurchase Agreements		$ 7,132,012
Weighted average rate		0.42%		0.38%
60 to 89 days | Agency Mortgage Backed Securities And Debentures [Member]
Repurchase Agreements:
Repurchase Agreements		$ 7,132,012		$ 4,040,677
90 to 119 days
Repurchase Agreements:
Repurchase Agreements		$ 1,507,387
Weighted average rate		0.43%		0.50%
90 to 119 days | Agency Mortgage Backed Securities And Debentures [Member]
Repurchase Agreements:
Repurchase Agreements		$ 1,507,387		$ 2,945,495
Over 120 days
Repurchase Agreements:
Repurchase Agreements	[2]	$ 17,498,675
Weighted average rate		1.32%	[2]	1.24%
Over 120 days | Agency Mortgage Backed Securities And Debentures [Member]
Repurchase Agreements:
Repurchase Agreements	[2]	$ 17,376,675		$ 18,685,118
Over 120 days | Commercial Loan [Member]
Repurchase Agreements:
Repurchase Agreements	[2]	$ 122,000

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

[2] Approximately 17% and 15% of the total repurchase agreements had a remaining maturity over 1 year as of June 30, 2015 and December 31, 2014, respectively.