Repurchase Agreements - Remaining Maturities, Collateral Types and Weighted Average Rate (Detail) - USD ($) $ in Thousands		Mar. 31, 2016		Dec. 31, 2015
Repurchase Agreements:
Repurchase agreements		$ 54,448,141		$ 56,230,860	[1]
Weighted Average Rate		0.99%		0.90%
Credit Risk Transfer Securities
Repurchase Agreements:
Repurchase agreements		$ 113,969		$ 142,853
Non Agency MBS
Repurchase Agreements:
Repurchase agreements		623,196		435,581
Commercial Loan [Member]
Repurchase Agreements:
Repurchase agreements		187,793		179,428
Agency Mortgage Backed Securities
Repurchase Agreements:
Repurchase agreements		53,523,183		$ 55,472,998
1 day
Repurchase Agreements:
Repurchase agreements		$ 8,050,000
Weighted Average Rate		0.62%		0.00%
1 day | Agency Mortgage Backed Securities
Repurchase Agreements:
Repurchase agreements		$ 8,050,000
2 to 29 days
Repurchase Agreements:
Repurchase agreements		$ 12,841,928		$ 20,467,487
Weighted Average Rate		0.75%		0.69%
2 to 29 days | Credit Risk Transfer Securities
Repurchase Agreements:
Repurchase agreements		$ 54,656		$ 83,664
2 to 29 days | Non Agency MBS
Repurchase Agreements:
Repurchase agreements		320,677		260,359
2 to 29 days | Agency Mortgage Backed Securities
Repurchase Agreements:
Repurchase agreements		12,466,595		20,123,464
30 to 59 days
Repurchase Agreements:
Repurchase agreements		$ 4,878,678		$ 8,023,209
Weighted Average Rate		0.82%		0.74%
30 to 59 days | Credit Risk Transfer Securities
Repurchase Agreements:
Repurchase agreements		$ 59,313		$ 59,189
30 to 59 days | Non Agency MBS
Repurchase Agreements:
Repurchase agreements		156,354		65,374
30 to 59 days | Agency Mortgage Backed Securities
Repurchase Agreements:
Repurchase agreements		4,663,011		7,898,646
60 to 89 days
Repurchase Agreements:
Repurchase agreements		$ 9,264,997		$ 4,125,426
Weighted Average Rate		0.96%		0.74%
60 to 89 days | Non Agency MBS
Repurchase Agreements:
Repurchase agreements		$ 146,165		$ 78,833
60 to 89 days | Agency Mortgage Backed Securities
Repurchase Agreements:
Repurchase agreements		9,118,832		4,046,593
90 to 119 days
Repurchase Agreements:
Repurchase agreements		$ 4,270,155		$ 4,846,580
Weighted Average Rate		0.95%		0.60%
90 to 119 days | Agency Mortgage Backed Securities
Repurchase Agreements:
Repurchase agreements		$ 4,270,155		$ 4,846,580
Over 120 days
Repurchase Agreements:
Repurchase agreements	[2]	$ 15,142,383		$ 18,768,158
Weighted Average Rate		1.46%	[2]	1.33%
Over 120 days | Non Agency MBS
Repurchase Agreements:
Repurchase agreements	[2]			$ 31,015
Over 120 days | Commercial Loan [Member]
Repurchase Agreements:
Repurchase agreements	[2]	$ 187,793		179,428
Over 120 days | Agency Mortgage Backed Securities
Repurchase Agreements:
Repurchase agreements	[2]	$ 14,954,590		$ 18,557,715

[1] Derived from the audited consolidated financial statements at December 31, 2015.

[2] Approximately 12% and 14% of the total repurchase agreements had a remaining maturity over 1 year as of March 31, 2016 and December 31, 2015, respectively.