Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions			3 Months Ended	9 Months Ended
	May 23, 2019	Feb. 01, 2019	Sep. 30, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Performance targets, maximum potential obligation			$ 28.9	$ 28.9		$ 14.0
Treasury securities			0.2	0.2		0.2
Held-to-maturity investments, carrying value			0.2	0.2		0.2
Held-to-maturity investments, fair value					$ 0.2	$ 0.2
Short-term investments			0.5	0.5
Purchase of short-term investments			235.0	$ 515.0
Sale of short-term investments			260.0
Revolving Credit Facility [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Basis spread on variable interest rate				1.125%
Revolving Credit Facility [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Basis spread on variable interest rate				1.75%
2018 Term Loan [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan period	4 years	4 years
Notional amount	$ 750.0	$ 1,200.0
Derivative basis spread on variable interest rate	2.00%	2.00%
Derivative fixed interest rate	4.08%	4.495%
Liability fair value of swaps			$ 62.4	$ 62.4