Derivative Financial Instruments - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Total amount of notional derivative contracts	$ 5,500,000
Net positive exposure	19,600	$ 44,600
Estimated reclassification of AOCI to interest expense	1,800
Cash collateral (held) pledged	0	0
Cash collateral held relative to derivative exposure	0	1,000
Cash collateral pledged	21,600	49,100
CME
Derivative [Line Items]
Total amount of notional derivative contracts	$ 4,800,000
Percentage of total notional derivative contracts	87.60%
LCH
Derivative [Line Items]
Total amount of notional derivative contracts	$ 700,000
Percentage of total notional derivative contracts	12.40%
Interest Rate Swaps
Derivative [Line Items]
Total amount of notional derivative contracts	$ 5,459,075	5,177,388
Net position	(1,772)	(18,129)
Fair Value | Designated as Hedging Instrument
Derivative [Line Items]
Net position	19,800	30,000
Fair Value | Interest Rate Swaps | Designated as Hedging Instrument
Derivative [Line Items]
Total amount of notional derivative contracts	3,062,849	3,628,062
Derivative instruments	1,800	18,100
Decrease in fair value of collateral	3,500
Variation margin	19,900
Net position	630	$ (2,590)
Fair Value | Contracts Other Than Chicago Mercantile Exchange | Designated as Hedging Instrument
Derivative [Line Items]
Decrease in fair value of collateral	3,500
Variation margin	25,400
Cash collateral (held) pledged	$ 32,100