Commitments and Contingencies Clean Air Act (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Obligation with Joint and Several Liability Arrangement [Line Items]
Supplemental environmental legal expense paid	$ 1.5
Supplemental environment projects legal expense	1.0
Injunctive relief legal expense accrual	18.0
Mitigation legal expense accrual	$ 1.7