Long-Term Debt (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Line of Credit Facility [Line Items]
Line of credit facility, covenant terms, Debt to EBITDA ratio	3.25
Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,250.0
Line of credit facility, margin for LIBOR-based loans, percentage	0.875%
Line of credit facility, margin for base-rate loans, percentage	0.00%
Line of credit facility commitment fee amount, percentage	0.125%
Line of credit facility, covenant terms, Coverage ratio	4.00
Line of Credit [Member] | London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Line of credit facility, basis spread on interest rate, percentage	1.00%
Line of Credit [Member] | Federal Funds Rate [Member]
Line of Credit Facility [Line Items]
Line of credit facility, basis spread on interest rate, percentage	0.50%
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Capacity Available for Trade Purchases	$ 11.3
Letters of credit, renewable term	1 year
Carrying Value [Member]
Line of Credit Facility [Line Items]
Long-term Debt	$ 2,093.6	$ 2,102.4
Carrying Value [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of credit facility, amount outstanding	19.4	28.7
Carrying Value [Member] | Unsecured Debt [Member]
Line of Credit Facility [Line Items]
Long-term Debt	$ 2,074.2
Due November 30, 2022 [Member] | Unsecured Debt [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.125%
Debt Instrument, Face Amount	$ 600.0
Due November 30, 2022 [Member] | Carrying Value [Member] | Unsecured Debt [Member]
Line of Credit Facility [Line Items]
Long-term Debt	$ 596.4	$ 596.3
Minimum [Member] | Senior Notes [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Term	5 years
Maximum [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Term	5 years