Commitments and Contingencies, Environmental Contingencies (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) Site	Dec. 31, 2016 USD ($)	Dec. 31, 2015
Clean Power Plan [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Length of Public Comment Period	60 days
National Ambient Air Quality Standards for Ozone [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of hours measured for standard			8
Former Level of Air Quality Concentrations (in parts per billion)			75
Revised Level of Air Quality Concentrations (in parts per billion)			70
Number of years for the state to achieve the designated attainment standard after revision to the standard	2 years
Number of years for the extension for the state to achieve the designated attainment standard	1 year
Other MGP Sites [Member]
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 | $	$ 3.3	$ 1.7