Debt (Narrative) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Less unamortized discount	$ (3.3)	$ (3.7)
Debt Maturity Period	5 years
Long-term Debt, Gross	$ 2,221.8	2,221.8
Principal amount of senior notes outstanding	$ 2,221.8
Debt Instrument, Maturity Date Range, Start	Sep. 01, 2016
Debt Instrument, Maturity Date Range, End	May 01, 2023
Senior Notes, stated interest rate, minimum (in hundredths)	5.25%
Senior Notes, stated interest rate, maximim (in hundredths)	6.875%
Revolving Credit Facility due 2019 [Member]
Debt Instrument [Line Items]
Maturity date of long-term debt	Dec. 02, 2019
Borrowing capacity	$ 1,800.0
Weighted-average interest rate (in hundredths)	2.20%
Letters of credit outstanding	$ 3.7	3.7
Long-term Debt, Gross	$ 0.0	0.0
6.05% Senior Notes due 2016 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate (in hundredths)	6.05%
Maturity date of long-term debt	Sep. 01, 2016
Long-term Debt, Gross	$ 176.8	176.8
6.80% Senior Notes due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate (in hundredths)	6.80%
Maturity date of long-term debt	Apr. 01, 2018
Long-term Debt, Gross	$ 134.0	134.0
6.80% Senior Notes Due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate (in hundredths)	6.80%
Maturity date of long-term debt	Mar. 01, 2020
Long-term Debt, Gross	$ 136.0	$ 136.0