Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Senior notes due 2017 [Member]
Fair value of senior notes
Fair value of senior notes	$ 258.1	$ 254.8	$ 254.7
Carrying value of senior notes	250.0	250.0	250.0
Senior notes due 2020 [Member]
Fair value of senior notes
Fair value of senior notes	244.4	239.6	242.3
Carrying value of senior notes	222.1	221.9	221.6
Senior notes due 2022 [Member]
Fair value of senior notes
Fair value of senior notes	424.0	252.8	0
Carrying value of senior notes	$ 405.4	$ 247.9	$ 0