Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional value	$ 5,800,000
Cash collateral (held) pledged	0	$ 0
Net derivatives	38,300	44,600
Cash collateral held relative to derivative exposure	900	1,000
Cash collateral pledged	48,500	49,100
Chicago Mercantile Exchange
Derivative [Line Items]
Notional value	$ 4,600,000
Percent of total notional derivative contracts	80.50%
London Clearing House
Derivative [Line Items]
Notional value	$ 700,000
Percent of total notional derivative contracts	12.50%
Interest rate swaps
Derivative [Line Items]
Notional value	$ 5,762,829	5,177,388
Net position	(10,926)	(18,129)
Designated as Hedging Instrument | Fair Value Hedges
Derivative [Line Items]
Net position	36,700	30,000
Designated as Hedging Instrument | Fair Value Hedges | Interest rate swaps
Derivative [Line Items]
Notional value	4,071,595	3,628,062
Derivative instruments	10,900	18,100
Increase (decrease) in fair value of collateral	5,100
Variation margin	12,300
Net position	(2,648)	$ (2,590)
Designated as Hedging Instrument | Fair Value Hedges | Contracts other than CME
Derivative [Line Items]
Increase (decrease) in fair value of collateral	5,100
Variation margin	17,800
Cash collateral (held) pledged	$ (6,000)