Commitments and Contingencies, Environmental Contingencies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) Site	Dec. 31, 2015 USD ($) Period
Implementation of the National Ambient Air Quality Standard for Sulfur Dioxide
Environmental Requirements [Abstract]
Number of Phases Under a Consent Decree Which the EPA is Requiring States to Evaluate Areas for Attainment	3
National Ambient Air Quality Standards for Ozone
Environmental Requirements [Abstract]
Number of Hours Measured for Standard | Period		8
Former Level of Air Quality Concentrations (in parts per billion)		75
Revised Level of Air Quality Concentrations (in parts per billion)		70
Minimum | Capital Commitments | Federal Clean Water Act Effluent Limitations Guidelines
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply With Regulation	$ 9.0
Maximum | Capital Commitments | Federal Clean Water Act Effluent Limitations Guidelines
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply With Regulation	$ 21.0
Other MGP Sites
Site Contingency [Line Items]
Number of Identified MGP Sites Under Current Investigation and/or Remediation | Site	2
Liability for Estimated Cost of Remediating Sites	$ 1.7	$ 1.7