Summary of Gross Amounts, Amounts Offset and Net Amounts of Repurchase Agreement and Reverse Repurchase Agreement (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017		Dec. 31, 2016
Disclosure of Repurchase Agreements [Abstract]
Gross amounts -reverse repurchase agreements	$ 1,875,000		$ 400,000
Amounts offset - reverse repurchase agreement	(1,875,000)		(400,000)
Netted amounts -reverse repurchase	0		0
Gross amounts -repurchase agreement	71,305,268		65,615,810
Amounts offset -repurchase agreement	(1,875,000)		(400,000)
Netted amounts -repurchase agreement	$ 69,430,268	[1]	$ 65,215,810	[2]

[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.