Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Senior notes due 2017 [Member]
Fair value of senior notes
Fair value of senior notes	$ 256.7	$ 258.1	$ 254.8
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	239.1	244.4	239.6
Carrying value of senior notes	222.4	222.1	221.9
Senior Notes due 2022 [Member]
Fair value of senior notes
Fair value of senior notes	409.0	424.0	252.8
Carrying value of senior notes	$ 404.9	$ 405.4	$ 247.9