Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional value	$ 5,900,000
Cash collateral (held) pledged	0	$ 0
Net derivatives	23,600	44,600
Cash collateral held relative to derivative exposure	300	1,000
Cash collateral pledged	31,300	49,100
Chicago Mercantile Exchange
Derivative [Line Items]
Notional value	$ 5,100,000
Percent of total notional derivative contracts	85.80%
London Clearing House
Derivative [Line Items]
Notional value	$ 700,000
Percent of total notional derivative contracts	11.90%
Interest rate swaps
Derivative [Line Items]
Notional value	$ 5,887,127	5,177,388
Net position	(8,000)	(18,129)
Designated as Hedging Instrument | Fair Value Hedges
Derivative [Line Items]
Net position	23,000	30,000
Designated as Hedging Instrument | Fair Value Hedges | Interest rate swaps
Derivative [Line Items]
Notional value	3,480,365	3,628,062
Derivative instruments	8,000	18,100
Increase (decrease) in fair value of collateral	2,000
Variation margin	12,100
Net position	(1,168)	$ (2,590)
Designated as Hedging Instrument | Fair Value Hedges | Contracts other than CME
Derivative [Line Items]
Increase (decrease) in fair value of collateral	2,000
Variation margin	17,600
Cash collateral (held) pledged	$ 22,600