Leases - Future Minimum Lease Payments (Details) $ in Millions	Dec. 31, 2019 USD ($)
Total operating leases
2020	$ 2.6
2021	0.7
2022	0.5
2023	0.5
2024	0.5
Thereafter	13.2
Total minimum lease payments	18.0
Less: interest	(7.4)
Present value of minimum lease payments	10.6
Less: short-term lease liabilities	(2.2)
Long-term lease liabilities	8.4
Total finance leases
2020	401.7
2021	402.3
2022	396.9
2023	392.6
2024	392.4
Thereafter	5,239.8
Total minimum lease payments	7,225.7
Less: interest	(4,384.8)
Present value of minimum lease payments	2,840.9
Less: short-term lease liabilities	(57.8)
Long-term lease liabilities	2,783.1
Power purchase commitment
Total finance leases
2020	8.8
2021	9.4
2022	4.2
2023	0.0
2024	0.0
Thereafter	0.0
Total minimum lease payments	22.4
Less: interest	(4.0)
Present value of minimum lease payments	18.4
Less: short-term lease liabilities	(6.3)
Long-term lease liabilities	12.1
Port Washington Generating Station
Total finance leases
2020	98.5
2021	98.5
2022	98.5
2023	98.5
2024	98.4
Thereafter	681.7
Total minimum lease payments	1,174.1
Less: interest	(556.3)
Present value of minimum lease payments	617.8
Less: short-term lease liabilities	(25.4)
Long-term lease liabilities	592.4
Elm Road Generating Station
Total finance leases
2020	294.4
2021	294.4
2022	294.2
2023	294.1
2024	294.0
Thereafter	4,558.1
Total minimum lease payments	6,029.2
Less: interest	(3,824.5)
Present value of minimum lease payments	2,204.7
Less: short-term lease liabilities	(26.1)
Long-term lease liabilities	$ 2,178.6