Fair Value of Financial Instruments (Schedule of Fair Values of Financial Assets and Liabilities Measured On A Nonrecurring Basis) (Details) (Unsecured Debt [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Carrying Amount [Member] | Senior Notes [Member] | 4.875% Senior Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes	$ 400.0	$ 0
Carrying Amount [Member] | Senior Notes [Member] | 7.625% Senior Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes	0	150.0
Carrying Amount [Member] | Convertible Debt [Member] | 2009 Senior Convertible Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior convertible notes	12.6	12.7
Carrying Amount [Member] | Convertible Debt [Member] | 2011 Senior Convertible Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior convertible notes	197.0	193.0
Fair Value [Member] | Senior Notes [Member] | 4.875% Senior Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes	378.0	0
Fair Value [Member] | Senior Notes [Member] | 7.625% Senior Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes	0	154.9
Fair Value [Member] | Convertible Debt [Member] | 2009 Senior Convertible Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior convertible notes	24.7	21.0
Fair Value [Member] | Convertible Debt [Member] | 2011 Senior Convertible Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior convertible notes	$ 397.8	$ 338.3