Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	Dec. 31, 2017 USD ($)
ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment	$ 52.0
ASU 2014-09 | Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment	52.0
Cumulative-effect adjustment, tax	15.6
ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment	67.9
ASU 2016-01 | Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment	$ 67.9