Leases - Right of Use Assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Right of use assets
Total finance lease right of use assets	$ 57.0	$ 40.6
Operating lease right of use assets	20.7	41.4
Power Purchase Contract
Right of use assets
Under finance lease	140.3	140.3
Accumulated amortization	(132.3)	(126.6)
Total finance lease right of use assets	8.0	13.7
Two Creeks
Right of use assets
Under finance lease	7.7	7.7
Accumulated amortization	(0.2)	(0.1)
Total finance lease right of use assets	7.5	7.6
Badger Hollow I
Right of use assets
Under finance lease	19.5	19.5
Accumulated amortization	(0.6)	(0.2)
Total finance lease right of use assets	18.9	19.3
Badger Hollow II
Right of use assets
Under finance lease	22.8	0.0
Accumulated amortization	(0.2)	0.0
Total finance lease right of use assets	$ 22.6	$ 0.0