Debt (Senior Secured Credit Facility) (Narrative) (Details) - USD ($) $ in Millions			3 Months Ended
	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2019
Debt Instrument [Line Items]
Line of credit facility, letters of credit issued	$ 141.7	$ 128.4	$ 141.7
Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	1,500.0		1,500.0
Line of credit facility, letters of credit issued	$ 101.5	$ 88.2	101.5
Line of credit facility, unused facility fee (percentage)	0.30%	0.35%
Credit Facility [Member] | Revolving Loans [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,100.0		1,100.0
Line of credit facility, amount of borrowings denominated in foreign currencies	114.0	$ 128.0	114.0
Line of credit facility, remaining borrowing capacity	654.2	554.9	654.2
Line of credit facility, remaining borrowing capacity available in either Canadian dollars or Mexican pesos	$ 185.7	$ 91.9	$ 185.7
Credit Facility [Member] | Revolving Loans [Member] | Weighted Average [Member]
Debt Instrument [Line Items]
Line of credit facility, interest rate (percentage)	3.92%	4.23%	3.92%
Credit Facility [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 400.0		$ 400.0
Line of credit facility, term loan, amount of quarterly principal installment payments			$ 5.0
Line of credit facility, interest rate (percentage)	4.03%	4.27%	4.03%
Credit Facility [Member] | Letters of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, capacity available for letters of credit	$ 548.5	$ 554.9	$ 548.5
Credit Facility [Member] | Letters of Credit [Member] | Performance Standby [Member]
Debt Instrument [Line Items]
Line of credit facility, interest rate (percentage)	0.75%	0.875%	0.75%
Credit Facility [Member] | Letters of Credit [Member] | Commercial and/or Financial Standby [Member]
Debt Instrument [Line Items]
Line of credit facility, interest rate (percentage)	1.625%	1.75%	1.625%