Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional value	$ 5,600,000
Cash collateral (held) pledged	320	$ 0
Net derivatives	43,200	44,600
Cash collateral held relative to derivative exposure	300	1,000
Cash collateral pledged	50,300	49,100
Chicago Mercantile Exchange
Derivative [Line Items]
Notional value	$ 4,500,000
Percent of total notional derivative contracts	79.80%
London Clearing House
Derivative [Line Items]
Notional value	$ 700,000
Percent of total notional derivative contracts	13.10%
Interest rate swaps
Derivative [Line Items]
Notional value	$ 5,671,270	5,177,388
Net position	(7,681)	(18,129)
Designated as Hedging Instrument | Fair Value Hedges
Derivative [Line Items]
Net position	42,600	30,000
Designated as Hedging Instrument | Fair Value Hedges | Interest rate swaps
Derivative [Line Items]
Notional value	3,988,051	3,628,062
Derivative instruments	7,600	18,100
Increase (decrease) in fair value of collateral	6,300
Variation margin	16,700
Net position	(416)	$ (2,590)
Designated as Hedging Instrument | Fair Value Hedges | Contracts other than CME
Derivative [Line Items]
Increase (decrease) in fair value of collateral	(6,300)
Variation margin	22,700
Cash collateral (held) pledged	$ (3,800)