Commitments and Contingencies, Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Capital Leases [Abstract]
Amortization expense under capital lease assets	$ 5.6	$ 5.3	$ 8.1
Property Held Under Capital Leases, Net [Abstract]
Property held under capital lease	221.2	221.2
Accumulated depreciation	(76.2)	(70.6)
Total property held under capital leases, net	145.0	150.6
Capital Leases, Future Minimum Payments Due [Abstract]
2019	24.9
2020	24.8
2021	23.6
2022	20.5
2023	20.3
Thereafter	420.4
Total minimum obligation	534.5
Interest component of obligation	(389.5)
Present value of minimum obligation	$ 145.0
Operating Leased Assets [Line Items]
Operating Lease Purchase Power Agreement Expiration Date	2034
Operating Leases, Future Minimum Payments Due [Abstract]
2019	$ 106.3
2020	106.6
2021	105.9
2022	91.0
2023	78.1
Operating Leases, Future Minimum Payments, Due Thereafter	$ 342.0
WYCO Totem Gas Storage Facilities
Capital Leases [Abstract]
Ownership interest in joint venture (in hundredths)	50.00%
Capital Lease Obligations	$ 120.0	123.8
Gas Storage Facilities
Property Held Under Capital Leases, Net [Abstract]
Property held under capital lease	200.5	200.5
Gas Pipeline
Property Held Under Capital Leases, Net [Abstract]
Property held under capital lease	20.7	20.7
Office Space and Other Equipment
Operating Leases [Abstract]
Total expenses under operating lease obligations	110.6	108.6	118.2
Operating Leases, Future Minimum Payments Due [Abstract]
2019	10.8
2020	10.7
2021	9.5
2022	8.4
2023	8.1
Operating Leases, Future Minimum Payments, Due Thereafter	53.4
Purchased Power Agreements
Operating Leases [Abstract]
Payments for capacity for PPAs under operating lease obligations	96.6	$ 96.1	$ 102.4
Operating Leases, Future Minimum Payments Due [Abstract]
2019	95.5
2020	95.9
2021	96.4
2022	82.6
2023	70.0
Operating Leases, Future Minimum Payments, Due Thereafter	$ 288.6