Fair Value of Financial Assets and Liabilities - Changes in Level 3 Commodity Derivatives (Details) - Commodity Contract - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs		$ (17.1)	$ 0.2	$ (17.1)	$ 0.2	$ (14.6)	$ (12.7)	$ (0.1)	$ 0.1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Settlements		(0.3)	0.0	(1.1)	(0.9)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	[1]	(2.2)	0.3	(3.3)	1.0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers, Net		$ 0.0	$ 0.0	$ 0.0	$ 0.0

[1] Amounts relate to commodity derivatives held at the end of the period.