Recent Authoritative Accounting Guidance - New Accounting Pronouncements and Changes in Accounting Principles (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reduction in income tax expense	$ 2.6	$ 0.0	$ 0.0
Stock Option Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reduction in income tax expense	2.0
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reduction in income tax expense	0.6
Accounting Standards Update 2018-02 | Adjustments for New Accounting Principle, Early Adoption
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stranded amounts resulting from remeasurement of deferred tax assets and liabilities	$ 3.1