Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ 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 Data [Abstract]
Operating revenues	$ 327.2	$ 294.8	$ 327.3	$ 345.9	$ 325.1	$ 277.9	$ 285.3	$ 335.4	$ 1,295.2	$ 1,223.7	$ 1,322.6
Operating expenses	216.4	207.4	204.9	192.8	217.8	197.1	199.6	194.7	821.5	809.2	858.6
Operating income	110.8	87.4	122.4	153.1	107.3	80.8	85.7	140.7	473.7	414.5	464.0
Interest expense, net	42.5	45.4	45.5	45.0	44.8	43.5	43.2	44.1
Other income	(1.2)	(0.6)		(0.2)	(0.7)	(0.7)		(0.8)
Other expense			1.1				0.2
Income before income taxes	69.5	42.6	75.8	108.3	63.2	38.0	42.3	97.4	296.2	240.9	298.0
Income taxes	0.1	0.1	0.1	0.2	0.2	0.1	0.1	0.2	0.5	0.6	1.0
Net income	$ 69.4	$ 42.5	$ 75.7	$ 108.1	$ 63.0	$ 37.9	$ 42.2	$ 97.2	$ 295.7	$ 240.3	$ 297.0