New Accounting Pronouncements (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings			$ 3,211.5	$ 2,820.8
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings			$ 3,211.5
Retained Earnings | Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassifications due to adoption of accounting standard		$ 6.5
Other Nonoperating Income (Expense) | Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassifications due to adoption of accounting standard	$ (0.6)	$ (1.2)