Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Selected quarterly financial information
Revenues	$ 283.5	$ 285.4	$ 289.7	$ 287.2	$ 285.4	$ 291.6	$ 291.3	$ 285.5	$ 1,145.8	$ 1,153.8	$ 1,188.1
Income (loss) from operations	36.5	46.7	47.0	46.2	48.1	57.1	47.9	(174.7)	176.4	(21.6)	170.7
Net income (loss)	$ 6.9	$ 11.4	$ 9.7	$ 8.4	$ 17.1	$ 31.4	$ 25.4	$ (161.2)	$ 36.4	$ (87.3)	$ 185.3
Basic net income (loss) per share (in dollars per share)	$ 0.08	$ 0.14	$ 0.12	$ 0.10	$ 0.21	$ 0.39	$ 0.31	$ (1.91)	$ 0.45	$ (1.07)	$ 2.35
Diluted net income (loss) per share (in dollars per share)	$ 0.08	$ 0.14	$ 0.12	$ 0.10	$ 0.21	$ 0.38	$ 0.31	$ (1.91)	$ 0.45	$ (1.07)	$ 2.35
Impairment	$ 9.7							$ 216.3	$ 9.7	$ 216.3	$ 147.4
Amount of out-of-quarter decrease in regulatory revenue due to change in contract terms	1.5
Increase in depreciation expense related to assets placed in service	$ 3.4
Errors in accounting for deferred income taxes | Adjustment
Selected quarterly financial information
Net income (loss)		$ 0.2	$ 0.3	$ 0.2	$ 0.8	$ 0.9	$ 0.8	$ 0.8		$ 3.3	$ 1.1
Basic and diluted net income (loss) per share (in dollars per share)					$ 0.01	$ 0.01	$ 0.01	$ 0.01