Fair Value Measurements - Financial Assets and Liabilities Not Carried at Fair Value (Details) - Level 2 - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes(1)	[1]	$ 745.8	$ 745.7
Convertible securities		310.5	309.9
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes(1)		753.4	765.2
Convertible securities		$ 534.2	$ 549.8

[1] The carrying value of the senior notes is reported in Senior debt.