Repurchase Agreements - Narrative (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Repurchase Agreements:
Repurchase agreements - outstanding	$ 54,448,141	$ 56,230,860	[1]
Repurchase agreements - weighted average borrowing rates	1.87%	1.83%
Repurchase agreements - weighted average remaining maturities (in days)	136 days	151 days
Other secured financing long term, amount	$ 3,588,326	$ 1,845,048	[1]
Secured financings and interest rate swaps - collateral held, estimated fair value	62,700,000	62,300,000
Secured financings and interest rate swaps - collateral held, accrued interest	$ 188,600	$ 171,700
FHLB De Moines [Member]
Repurchase Agreements:
Debt weighted average interest rate	0.59%	0.59%
FHLB De Moines [Member] | Matured Within 90 Days [Member]
Repurchase Agreements:
Other secured financing long term, amount		$ 402,800
Other secured financing short term amount, expiration period		90 days
FHLB De Moines [Member] | Maturity Period Beyond Three Years [Member]
Repurchase Agreements:
Other secured financing long term, amount	$ 3,600,000	$ 1,400,000
Other secured financing short term amount, expiration period	3 years	3 years

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