Share-Based Compensation - Performance and Market-Based Award Vesting and Unrecognized Compensation Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Performance Based Awards
Share-based Compensation
Unrecognized compensation cost (in dollars)	$ 28.2
Performance Based Awards | Maximum
Share-based Compensation
Vesting percentage		200.00%
Market Based Share Awards
Share-based Compensation
Unrecognized compensation cost (in dollars)	$ 4.8
Market Based Share Awards | Maximum
Share-based Compensation
Vesting percentage	200.00%