Accounting Policies - Recently Adopted Accounting Standards (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Jan. 01, 2017
New Accounting Pronouncements
Tax benefit of stock awards exercised	$ 0.4	$ 0.3
Non-current deferred tax asset	3.0		$ 1.6
Noncurrent deferred tax liability	48.6		55.2
Long-term Debt	653.6		499.5
Unamortized debt issuance costs	3.2		$ 2.4
Previously Reported
New Accounting Pronouncements
Current deferred tax assets	38.6
Non-current deferred tax asset	1.5
Noncurrent deferred tax liability	85.7
Accounting Standards Update 2015-17 | Adjustment
New Accounting Pronouncements
Non-current deferred tax asset	1.5
Noncurrent deferred tax liability	38.6
Accounting Standards Update 2016-09
New Accounting Pronouncements
Cumulative effect adjustment				$ 0.5
Tax benefit of stock awards exercised	$ 0.4