Stock-Based Compensation - Summary of Stock Options Outstanding (Details) - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding By Exercise Price [Line Items]
Options Outstanding, Shares	2.3	2.6	2.1	2.0
Options Outstanding, Weighted Average Remaining life	6 years 2 months 12 days
Options Outstanding, Weighted Average Exercise Price (usd per share)	$ 11.08	$ 10.71	$ 10.29	$ 9.28
Options Exercisable, Shares	1.6	1.6	1.4
Options Exercisable, Weighted Average Exercise Price (usd per share)	$ 9.57	$ 8.96	$ 8.47
$5.01 — $10.00
Schedule Of Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding By Exercise Price [Line Items]
Range lower (usd per share)	5.01
Range upper (usd per share)	$ 10.00
Options Outstanding, Shares	1.0
Options Outstanding, Weighted Average Remaining life	4 years
Options Outstanding, Weighted Average Exercise Price (usd per share)	$ 6.63
Options Exercisable, Shares	1.0
Options Exercisable, Weighted Average Exercise Price (usd per share)	$ 6.63
10.01 — 15.00
Schedule Of Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding By Exercise Price [Line Items]
Range lower (usd per share)	10.01
Range upper (usd per share)	$ 15.00
Options Outstanding, Shares	0.8
Options Outstanding, Weighted Average Remaining life	7 years 6 months
Options Outstanding, Weighted Average Exercise Price (usd per share)	$ 13.02
Options Exercisable, Shares	0.4
Options Exercisable, Weighted Average Exercise Price (usd per share)	$ 13.30
15.01 — 20.00
Schedule Of Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding By Exercise Price [Line Items]
Range lower (usd per share)	15.01
Range upper (usd per share)	$ 20.00
Options Outstanding, Shares	0.5
Options Outstanding, Weighted Average Remaining life	8 years 1 month 6 days
Options Outstanding, Weighted Average Exercise Price (usd per share)	$ 16.51
Options Exercisable, Shares	0.2
Options Exercisable, Weighted Average Exercise Price (usd per share)	$ 16.09