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Stock-Based Compensation - Stock Option Activity (Details)
$ / shares in Units, shares in Thousands, $ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
shares
Shares  
Outstanding at beginning of period (in shares) | shares 1,526
Granted (in shares) | shares 558
Exercises (in shares) | shares (136)
Forfeitures (in shares) | shares (278)
Outstanding at end of period (in shares) | shares 1,670
Vested and exercisable (in shares) | shares 949
Weighted- Average Exercise Price  
Outstanding at beginning of period (in dollars per share) | $ / shares $ 37.42
Granted (in dollars per share) | $ / shares 37.59
Exercises (in dollars per share) | $ / shares 34.80
Forfeitures (in dollars per share) | $ / shares 40.38
Outstanding at end of period (in dollars per share) | $ / shares 37.20
Vested and exercisable (in dollars per share) | $ / shares $ 36.23
Weighted-Average Remaining Contractual Term and Aggregate Intrinsic Value  
Outstanding, Weighted-Average Remaining Contractual Term 6 years 11 months
Outstanding, Aggregate Intrinsic Value | $ $ 15.0
Vested and exercisable, Weighted-Average Remaining Contractual Term 5 years 8 months 12 days
Vested and exercisable, Aggregate Intrinsic Value | $ $ 9.4