Commitments and Contingencies, Leases (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Leased Assets [Line Items]
Operating Lease Purchase Power Agreement Expiration (year)		2033
Operating Leases, Future Minimum Payments Due [Abstract]
2016		$ 55.2
2017		53.0
2018		53.3
2019		53.3
2020		53.1
Thereafter		679.0
Office Space and Other Equipment
Operating Leases [Abstract]
Total expenses under operating lease obligations		54.5	$ 63.1	$ 64.2
Operating Leases, Future Minimum Payments Due [Abstract]
2016		3.7
2017		4.5
2018		4.7
2019		4.7
2020		4.5
Thereafter		61.7
Purchased Power Agreements
Operating Leases [Abstract]
Payments for capacity for PPAs under operating lease obligations		48.6	$ 57.1	$ 59.0
Operating Leases, Future Minimum Payments Due [Abstract]
2016	[1],[2]	51.5
2017	[1],[2]	48.5
2018	[1],[2]	48.6
2019	[1],[2]	48.6
2020	[1],[2]	48.6
Thereafter	[1],[2]	$ 617.3

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

[2] PPA operating leases contractually expire through 2033.