Commitments and Contingencies, Environmental Contingencies (Details) $ in Millions	Mar. 31, 2019 USD ($) Site	Dec. 31, 2018 USD ($)
Other MGP, Landfill, or Disposal Sites [Domain]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of identified MGP sites under current investigation and/or remediation | Site	3
Accrual for Environmental Loss Contingencies, Gross | $	$ 0.9	$ 0.6
Federal Coal Ash Regulation [Domain]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of sites where statistically significant increases over established groundwater standards exist	2
Number of regulated ash units that will still be in operation by the end of 2019	6