New Accounting Standards (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service cost	$ 2.8	$ 2.7
Net periodic benefit cost	1.2	1.9
Income tax expense	12.5	2.2
Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax expense	2.1	15.9
Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net benefit cost components other than service cost	1.6	$ 0.8
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating leases, Future minimum payments due	$ 76.9		$ 79.1