Derivative Financial Instruments - Additional Information (Details) - USD ($)	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional value	$ 9,000,000,000.0
Net derivatives	66,000,000	$ 27,000,000
Estimated accumulated other comprehensive income to be reclassified in next 12 months	2,400,000
Cash collateral held relative to derivative exposure	0	0
Cash collateral pledged	65,000,000	$ 27,000,000
CHICAGO MERCANTILE EXCHANGE
Derivative [Line Items]
Notional value	$ 8,500,000,000
Percent of total notional derivative contracts	94.00%
Amount of variation margin included as settlement	$ (106,400,000)
London Clearing House
Derivative [Line Items]
Notional value	$ 500,000,000
Percent of total notional derivative contracts	6.00%
Amount of variation margin included as settlement	$ 8,500,000