Long-Term Debt and Capital Lease Obligations (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) Megawatt	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Aug. 01, 2016 USD ($)
Long-term debt outstanding maturities
2017	$ 0.0
2018	250.0
2019	250.0
2020	0.0
2021	300.0
Thereafter	1,887.0
Total	2,687.0
Long-Term Debt and Capital Lease Obligations (Textuals) [Abstract]
Long-term Pollution Control Bond	80.0			$ 67.0
Total capital lease obligation	$ 2,785.0	$ 2,816.1
Number of PWGS natural gas-fired generation units	2
Total lease payment	$ 449.8	446.7	$ 411.9
Summary of capitalized leased facilities
Total leased facilities	2,060.2	2,141.7
Future minimum lease payments under capital lease and present value of net minimum lease payments
2017	432.0
2018	432.8
2019	433.6
2020	434.5
2021	435.3
Thereafter	6,856.5
Total minimum lease payments	9,024.7
Less: Estimated executory costs	(39.9)
Net minimum lease payments	8,984.8
Less: Interest	(6,199.8)
Present value of minimum lease payments	2,785.0
Less: Due currently	(28.5)	(123.6)
Long-term obligations under capital lease	$ 2,756.5	2,692.5
Purchase power agreement
Long-Term Debt and Capital Lease Obligations (Textuals) [Abstract]
Period of Power purchase contract with an unaffiliated independent power producer	25 years
Power capacity from a gas-fired cogeneration facility under capital lease (MW) | Megawatt	236
Minimum energy requirement in gas-fired cogeneration facility	0
Power purchase contract expiration year	Dec. 31, 2022
Power purchase contract expected future renewable period	10 years
Increase In Regulatory Asset due to Minimum lease payment	$ 78.5
Regulatory asset value at the end of life of contract	0.0
Total capital lease obligation	29.6
Power Purchase Commitment
Long-Term Debt and Capital Lease Obligations (Textuals) [Abstract]
Total lease payment	37.6	36.2	34.9
Summary of capitalized leased facilities
Capital lease asset	140.3	140.3
Accumulated amortization	(109.5)	(103.9)
Total leased facilities	30.8	36.4
Future minimum lease payments under capital lease and present value of net minimum lease payments
2017	13.9
2018	14.7
2019	15.5
2020	16.4
2021	17.2
Thereafter	7.6
Total minimum lease payments	85.3
Less: Estimated executory costs	(39.9)
Net minimum lease payments	45.4
Less: Interest	(15.8)
Present value of minimum lease payments	29.6
Less: Due currently	(2.7)
Long-term obligations under capital lease	$ 26.9
PWGS Units 1 and 2
Long-Term Debt and Capital Lease Obligations (Textuals) [Abstract]
Period of Power purchase contract with an unaffiliated independent power producer	25 years
Power capacity from a gas-fired cogeneration facility under capital lease (MW) | Megawatt	545
Capital lease obligation at the end of life of contract	$ 0.0
Total lease payment	82.4	103.8	99.2
Summary of capitalized leased facilities
Capital lease asset	704.2	692.5
Accumulated amortization	(274.7)	(245.7)
Total leased facilities	429.5	446.8
Future minimum lease payments under capital lease and present value of net minimum lease payments
2017	102.7
2018	102.7
2019	102.7
2020	102.7
2021	102.7
Thereafter	1,020.2
Total minimum lease payments	1,533.7
Less: Estimated executory costs	0.0
Net minimum lease payments	1,533.7
Less: Interest	(897.6)
Present value of minimum lease payments	636.1
Less: Due currently	(13.9)
Long-term obligations under capital lease	622.2
PWGS 1
Long-Term Debt and Capital Lease Obligations (Textuals) [Abstract]
Estimated Total Increase In Regulatory Asset due to Minimum Lease Payment, Over Life of Contract	$ 130.8
Increase In Regulatory Asset Due To Minimum Lease Payment, End Date of Increase	Dec. 31, 2021
PWGS 2
Long-Term Debt and Capital Lease Obligations (Textuals) [Abstract]
Power capacity from a gas-fired cogeneration facility under capital lease (MW) | Megawatt	545
Regulatory asset value at the end of life of contract	$ 0.0
Estimated Total Increase In Regulatory Asset due to Minimum Lease Payment, Over Life of Contract	$ 131.6
Increase In Regulatory Asset Due To Minimum Lease Payment, End Date of Increase	Dec. 31, 2024
ERGS
Long-Term Debt and Capital Lease Obligations (Textuals) [Abstract]
Period of Power purchase contract with an unaffiliated independent power producer	30 years
Regulatory asset value at the end of life of contract	$ 0.0
Capital lease obligation at the end of life of contract	0.0
Total lease payment	329.8	306.7	$ 277.8
Summary of capitalized leased facilities
Capital lease asset	2,053.5	2,043.9
Accumulated amortization	(453.6)	(385.4)
Total leased facilities	1,599.9	$ 1,658.5
Future minimum lease payments under capital lease and present value of net minimum lease payments
2017	315.4
2018	315.4
2019	315.4
2020	315.4
2021	315.4
Thereafter	5,828.7
Total minimum lease payments	7,405.7
Less: Estimated executory costs	0.0
Net minimum lease payments	7,405.7
Less: Interest	(5,286.4)
Present value of minimum lease payments	2,119.3
Less: Due currently	(11.9)
Long-term obligations under capital lease	2,107.4
ER 1
Long-Term Debt and Capital Lease Obligations (Textuals) [Abstract]
Estimated Total Increase In Regulatory Asset due to Minimum Lease Payment, Over Life of Contract	$ 542.8
Increase In Regulatory Asset Due To Minimum Lease Payment, End Date of Increase	May 31, 2029
ER 2
Long-Term Debt and Capital Lease Obligations (Textuals) [Abstract]
Estimated Total Increase In Regulatory Asset due to Minimum Lease Payment, Over Life of Contract	$ 447.2
Increase In Regulatory Asset Due To Minimum Lease Payment, End Date of Increase	Jan. 31, 2030