Share-Based Compensation - Stock Options (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Options outstanding and exercisable under the Company's 1991 Equity and Performance Incentive Plan and 2017 Equity and Performance Incentive Plan
Outstanding at January 1, 2019 | shares	2.5
Outstanding at January 1, 2019 | $ / shares	$ 27.05
Expired or forfeited | shares	0.1
Expired or forfeited | $ / shares	$ 29.76
Granted | shares	1.1
Granted | $ / shares	$ 4.33
Outstanding at March 31, 2019 | shares	3.5
Outstanding at March 31, 2019 | $ / shares	$ 19.83
Outstanding, weighted-average remaining contractual term	6 years
Outstanding, aggregate intrinsic value | $	$ 12.1
Options exercisable at March 31, 2019 | shares	1.9
Options exercisable at March 31, 2019 | $ / shares	$ 28.67
Options exercisable, weighted-average remaining contractual term	9 years
Options exercisable, aggregate intrinsic value | $	$ 15.4
Options vested and expected to vest(2) at March 31, 2019 | shares	3.5
Options vested and expected to vest(2) at March 31, 2019 | $ / shares	$ 19.83
Options vested and expected to vest, weighted-average remaining contractual term	8 years
Options vested and expected to vest, aggregate intrinsic value | $	$ 27.5