Secured Financing - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017		Dec. 31, 2016
Repurchase Agreements:
Repurchase agreements - outstanding	$ 69,430,268	[1]	$ 65,215,810	[2]
Repurchase agreements - weighted average effective borrowing rates	1.88%		1.64%
Repurchase agreements - weighted average remaining maturities (in days)	65 days		96 days
Other secured financing long term, amount	$ 3,713,256	[1]	$ 3,884,708	[2]
Secured financings and interest rate swaps - collateral held, estimated fair value	77,600,000		74,300,000
Secured financings and interest rate swaps - collateral held, accrued interest	$ 236,200		229,200
Minimum
Repurchase Agreements:
Other secured financing amount, long term, expiration period	3 years
Maximum
Repurchase Agreements:
Other secured financing amount, long term, expiration period	8 years
Maturity Period Beyond Three Years
Repurchase Agreements:
Other secured financing long term, amount	$ 90,000		$ 3,600,000
Other secured financing amount, long term, expiration period	3 years		3 years
Maturity Period Beyond Three Years | Minimum
Repurchase Agreements:
Other secured financing amount, long term, expiration period	1 year
Maturity Period Beyond Three Years | Maximum
Repurchase Agreements:
Other secured financing amount, long term, expiration period	3 years
FHLB De Moines
Repurchase Agreements:
Debt weighted average interest rate	1.41%		0.76%
FHLB De Moines | Maturity Period Beyond Three Years
Repurchase Agreements:
Other secured financing long term, amount	$ 3,500,000

[1] As a result of a change to a clearing organization's rulebook effective January 3, 2017, beginning with the first quarter 2017 and in subsequent periods the Company is presenting the fair value of centrally cleared interest rate swaps net of variation margin pledged under such transactions. The variation margin was previously reported under cash and cash equivalents and is currently reported as a reduction to interest rate swaps, at fair value. Balances reported prior to the effective date will not be adjusted.

[2] Derived from the audited consolidated financial statements at December 31, 2016.