Summary of Significant Accounting Policies (Narrative) (Details) - ASU 2014-09 $ in Millions	Dec. 31, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment	$ 52.0
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment	52.0
Cumulative-effect adjustment, tax	$ 15.6