Commitments and Contingencies, Environmental Contingencies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) Site	Oct. 30, 2015 Period	Dec. 31, 2014 USD ($)
Regional Haze Rules
Environmental Requirements [Abstract]
Amount spent on installation of emission controls	$ 75.2
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
Number of months in which the state would have to submit an implementation plan for the respective nonattaiment areas	18 months
Number of years for the state to achieve the designated attainment standard	5 years
National Ambient Air Quality Standards for Ozone
Environmental Requirements [Abstract]
Number of hours measured for standard | Period		8
Current level of air quality concentrations (in parts per billion)		75
Proposed 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		$ 1.8