Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets:
Derivatives assets	$ 6.9	$ 2.4
Fair Value, Measurements, Recurring
Assets:
Total assets	6.9	2.4
Fair Value, Measurements, Recurring | Commodity
Assets:
Derivatives assets	6.9	2.4
Fair Value, Measurements, Recurring | Level 1
Assets:
Total assets	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | Commodity
Assets:
Derivatives assets	0.0	0.0
Fair Value, Measurements, Recurring | Level 2
Assets:
Total assets	0.0	0.0
Fair Value, Measurements, Recurring | Level 2 | Commodity
Assets:
Derivatives assets	0.0	0.0
Fair Value, Measurements, Recurring | Level 3
Assets:
Derivatives assets	6.9
Total assets	6.9	2.4
Fair Value, Measurements, Recurring | Level 3 | Commodity
Assets:
Derivatives assets	$ 6.9	$ 2.4