Commitments and Contingencies, Leases (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012
Operating Leases, Future Minimum Payments Due [Abstract]
2015	$ 55.3
2016	52.4
2017	51.4
2018	51.0
2019	50.9
Thereafter	683.2
Purchased Power Agreements (PPAs)
Operating Leases [Abstract]
Payments for capacity for PPAs under operating lease obligations	57.1		59.0	56.0
Operating Leases, Future Minimum Payments Due [Abstract]
2015	52.0	[1],[2]
2016	49.0	[1],[2]
2017	49.0	[1],[2]
2018	49.0	[1],[2]
2019	49.0	[1],[2]
Thereafter	671.8	[1],[2]
Office Space and Other Equipment
Operating Leases [Abstract]
Total expenses under operating lease obligations	63.1		64.2	59.9
Operating Leases, Future Minimum Payments Due [Abstract]
2015	3.3
2016	3.4
2017	2.4
2018	2.0
2019	1.9
Thereafter	$ 11.4
Maximum
Operating Leased Assets [Line Items]
Purchase Power Agreement Duration Operating Lease, Maximum	2033

[1] PPA operating leases contractually expire through 2033.

[2] Amounts do not include PPAs accounted for as executory contracts.