Fair Value Measurements (Details) - Senior Notes [Member] - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Senior Notes Due 2017 [Member]
Fair value of senior notes
Fair value of senior notes	$ 23.0	$ 153.0	$ 256.7
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	21.4	133.1	239.1
Carrying value of senior notes	225.0	222.8	222.4
Senior Notes Due 2022 [Member]
Fair value of senior notes
Fair value of senior notes	34.5	198.0	409.0
Carrying value of senior notes	$ 400.0	$ 404.5	$ 404.9