Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Data [Abstract]
Operating revenues	$ 337.5	$ 300.5	$ 317.6	$ 367.0	$ 352.6	$ 303.3	$ 306.3	$ 345.0	$ 1,322.6	$ 1,307.2	$ 1,249.2
Operating expenses	213.8	189.7	250.4	202.2	219.7	209.6	197.0	203.4	856.1	829.7	853.4
Operating income	123.7	110.8	67.2	164.8	132.9	93.7	109.3	141.6	466.5	477.5	395.8
Interest expense, net	39.8	41.0	43.7	46.1	46.3	48.3	45.3	42.5
Other income	(0.4)	(0.3)	(0.6)	(0.8)	(1.8)	(1.9)	(1.9)	(2.1)
Income before income taxes	84.3	70.1	24.1	119.5	88.4	47.3	65.9	101.2	298.0	302.8	222.5
Income taxes	0.1	0.3	0.4	0.2	0.2	0.0	0.2	0.2	1.0	0.6	0.5
Net income	$ 84.2	$ 69.8	$ 23.7	$ 119.3	$ 88.2	$ 47.3	$ 65.7	$ 101.0	$ 297.0	$ 302.2	$ 222.0
Net income per common unit (in dollars per unit)	$ 0.33	$ 0.27	$ 0.09	$ 0.47	$ 0.34	$ 0.19	$ 0.26	$ 0.40	$ 1.16	$ 1.18	$ 0.87