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Schedule III - Real Estate and Accumulated Depreciation - Reconciliations (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Reconciliation of Real Estate      
Balance at beginning of year $ 1,619.3 $ 1,447.7 $ 1,325.1
Additions and improvements 20.4 232.8 317.8
Dispositions, retirements and other adjustments (14.3) (61.2) (194.7)
Impairment of assets 0.0 0.0 (0.5)
Balance at end of year 1,625.4 1,619.3 1,447.7
Reconciliation of Accumulated Depreciation      
Balance at beginning of year 127.5 107.6 133.5
Depreciation expense 27.4 24.3 20.4
Dispositions, retirements and other adjustments (0.5) (4.4) (46.3)
Balance at end of year $ 154.4 $ 127.5 $ 107.6