Commitments and Contingencies Long Term Supply and Capacity Purchase Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Long-term Purchase Commitment [Line Items]
Long term purchase committments costs incurred	$ 209.3	$ 228.4	$ 216.8
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
Purchase obligation due in next twelve months	197.0
Purchase obligation due in second year	149.6
Purchase obligation due in third year	124.3
Purchase obligation due in fourth year	126.9
Purchase obligation due in fifth year	122.1
Purchase obligation due thereafter	$ 1,300.0
Minimum [Member]
Long-term Purchase Commitment [Line Items]
Long term purchase commitments term	1 year
Maximum [Member]
Long-term Purchase Commitment [Line Items]
Long term purchase commitments term	25
Hydroelectric License Commitments [Member]
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
Hydro MOU commitment	$ 18.0
Hydroelectric License Commitments [Member] | Minimum [Member]
Long-term Purchase Commitment [Line Items]
Long term purchase commitments term	2019
Hydroelectric License Commitments [Member] | Maximum [Member]
Long-term Purchase Commitment [Line Items]
Long term purchase commitments term	2040