Commitments and Contingencies Commitments and Contingencies, Leases - Right-of-Use Assets (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross	$ 652.8
Operating Lease, Right-of-Use Asset, Accumulated Depreciation	(19.8)
Operating Lease, Right-of-Use Asset	633.0	$ 0.0
Finance Lease, Assets and Liabilities, Lessee [Abstract]
Finance Lease, Right-of-Use Asset, Gross	221.2
Finance Lease, Right-of-Use Asset, Accumulated Depreciation	(77.7)
Finance Lease, Right-of-Use Asset	143.5
PPAs
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross	585.1
Other
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross	67.7
Gas storage facilities
Finance Lease, Assets and Liabilities, Lessee [Abstract]
Finance Lease, Right-of-Use Asset, Gross	200.5
Gas pipeline
Finance Lease, Assets and Liabilities, Lessee [Abstract]
Finance Lease, Right-of-Use Asset, Gross	$ 20.7