Quarterly Financial Data (Unaudited) - Summary of Quarterly Results of Operations (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
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 400.4	$ 419.5	$ 419.1	$ 384.0	$ 394.8	$ 400.6	$ 398.1	$ 364.7	$ 1,623.0	$ 1,558.2	$ 1,507.6
Income from operations	18.4	29.5	8.7	17.9	29.2	17.5	35.0	20.0	$ 74.5	$ 101.7	$ 90.1
Consolidated net (loss) income	$ (3.2)	$ 3.6	$ (1.0)	$ (6.0)	$ 2.5	$ (4.9)	$ 9.7	$ 2.1
Basic (loss) income per share (in dollars per share)	$ (0.04)	$ 0.04	$ (0.01)	$ (0.07)	$ 0.03	$ (0.06)	$ 0.11	$ 0.02	$ (0.07)	$ 0.11	$ 0.43
Diluted (loss) income per share (in dollars per share)	$ (0.04)	$ 0.04	$ (0.01)	$ (0.07)	$ 0.03	$ (0.06)	$ 0.11	$ 0.02	$ (0.07)	$ 0.11	$ 0.43