Quarterly Financial Information - Unaudited (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues	$ 364.7	$ 142.3	$ 199.3	$ 428.6	$ 373.2	$ 157.9	$ 201.3	$ 456.9	$ 1,134.9	$ 1,189.3	$ 1,220.0
Operating income	89.8	62.3	75.3	147.0	111.7	68.9	76.5	148.3	374.4	405.4	305.8
Net income	50.9	32.6	40.6	84.6	$ 62.5	$ 38.6	$ 40.3	$ 85.1	208.7	226.5	161.2
Asset impairment charge, before income taxes									12.5	2.0	80.6
Pension settlement costs	$ 16.7	$ 0.0	$ 0.0	$ 0.0					$ 16.7	$ 0.0	$ 0.0
Basic earnings per common share	$ 0.29	$ 0.18	$ 0.23	$ 0.48	$ 0.36	$ 0.22	$ 0.23	$ 0.48	$ 1.18	$ 1.29	$ 0.92
Diluted earnings per common share	$ 0.29	$ 0.18	$ 0.23	$ 0.48	$ 0.35	$ 0.22	$ 0.23	$ 0.48	$ 1.18	$ 1.29	$ 0.92
Wexpro [Member]
Asset impairment charge, before income taxes	$ 12.1	$ 0.0	$ 0.0	$ 0.0					$ 12.1