Secured Financing - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Repurchase Agreements:
Repurchase agreements - outstanding	$ 65,215,810	$ 56,230,860
Repurchase agreements - weighted average effective borrowing rates	1.64%	1.83%
Repurchase agreements - weighted average remaining maturities (in days)	96 days	151 days
Other secured financing long term, amount	$ 3,884,708	$ 1,845,048
Secured financings and interest rate swaps - collateral held, estimated fair value	74,300,000	62,300,000
Secured financings and interest rate swaps - collateral held, accrued interest	$ 229,200	$ 171,700
FHLB De Moines
Repurchase Agreements:
Debt weighted average interest rate	0.76%	0.59%
FHLB De Moines | Maturity Period Beyond Three Years
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
FHLB De Moines | Matured Within 90 Days
Repurchase Agreements:
Other secured financing long term, amount		$ 402,800,000
Other secured financing short term amount, expiration period		90 days