Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) $ in Thousands	1 Months Ended	9 Months Ended
	May. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) Site Parties	Dec. 31, 2014 USD ($)
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Liability for estimated cost of remediating sites, current		$ 16,689	$ 29,116
Ashland MGP Site
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Number of properties not owned included in superfund site | Site		2
Accrual for Environmental Loss Contingencies, Gross		$ 95,700	107,600
Number of PRPs that have reached a settlement in principle | Parties		2
Liability for estimated cost of remediating sites, current		$ 16,600	$ 28,900
Amortization period for recovery of remediation costs in natural gas rates, low end of range (in years)		4 years
Amortization period for recovery of remediation costs in natural gas rates, high end of range (in years)		6 years
Public Utilities, Annual recovery collected through base rates	$ 4,700
Ashland MGP Site - Phase I Project Area
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross		$ 57,000
Estimated amount spent on cleanup		$ 39,000
Approved amortization period for recovery of remediation costs in natural gas rates (in years)		10 years
Carrying cost percentage to be applied to the unamortized regulatory asset for MGP remediation (in hundredths)		3.00%
Approved increase (decrease) in amortization expense granted by a regulatory body		$ 1,100
Ashland MGP Site - Sediments
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Estimated cost of remediating site, low end of range		63,000
Estimated cost of remediating site, high end of range		$ 77,000
Potential percent of increase to the high end of the range of estimated site remediation costs (in hundredths)		50.00%
Potential percent of decrease to the low end of the range of estimated site remediation costs (in hundredths)		30.00%
Railroad PRPs [Member] | Ashland MGP Site
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Site Contingency, Recovery from Third Party of Environmental Remediation Cost		$ 10,500
LE Myers Co. [Member] | Ashland MGP Site
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Site Contingency, Recovery from Third Party of Environmental Remediation Cost		$ 5,400
PSCW Proceeding - Electric and Gas Rate Case 2016 - Gas Rates 2016 [Member] | Ashland MGP Site
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Public Utilities, Requested annual recovery collected through base rates	$ 7,600