Quarterly Financial Information - Unaudited (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Selected Quarterly Financial Information [Abstract]
Revenues	$ 397.9	$ 149.6	$ 195.6	$ 476.9	$ 343.0	$ 141.8	$ 188.4	$ 425.7	$ 1,220.0	$ 1,098.9	$ 1,194.4
Operating income (loss)	122.6	(19.4)	71.2	131.4	108.6	63.6	72.6	130.9	305.8	375.7	366.9
Net income (loss)	68.1	(19.2)	39.4	72.9	63.8	33.8	39.2	75.2	161.2	212.0	207.9
Asset impairment charge, before income taxes	$ 0	$ 80.6	$ 0	$ 0					$ 80.6	$ 0	$ 0
Basic earnings (loss) per common share	$ 0.39	$ (0.11)	$ 0.22	$ 0.42	$ 0.36	$ 0.20	$ 0.22	$ 0.42	$ 0.92	$ 1.20	$ 1.17
Diluted earnings (loss) per common share	$ 0.39	$ (0.11)	$ 0.22	$ 0.41	$ 0.36	$ 0.19	$ 0.22	$ 0.42	$ 0.92	$ 1.19	$ 1.16