Long Term Debt (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Jun. 10, 2014	Jul. 26, 2012
Debt Instrument [Line Items]
Debt Instrument, Convertible, Carrying Amount of Equity Component			$ 37,700,000
Long-term Debt	595,266,000	593,350,000
Long-term Debt, Current Maturities	0
Total long-term debt, less current portion	595,266,000
Long-term Debt, Fair Value	732,800,000	711,100,000
Interest Paid	10,000,000
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Issuance Date	Jul. 26, 2012
Debt Instrument, Face Amount	250,000,000
Debt Instrument, Interest Rate, Stated Percentage	8.00%
Debt Instrument, Maturity Date	Aug. 01, 2020
Proceeds from Debt, Net of Issuance Costs	245,000,000
Debt Instrument, Unamortized Discount	3,700,000			5,000,000
Unamortized Debt Issuance Expense	1,100,000
Debt Instrument, Frequency of Periodic Payment	semi-annually
Debt Instrument, Date of First Required Payment	Feb. 01, 2013
Debt Instrument, Call Date, Earliest	Aug. 01, 2016
Long-term Debt	246,324,000
Long-term Debt, Fair Value	265,400,000
Convertible Debt Securities [Member]
Debt Instrument [Line Items]
Debt Instrument, Issuance Date	Jun. 10, 2014
Debt Instrument, Face Amount	402,500,000
Debt Instrument, Interest Rate, Stated Percentage	3.25%
Debt Instrument, Maturity Date	Jun. 15, 2029
Proceeds from Debt, Net of Issuance Costs	391,400,000
Debt Instrument, Unamortized Discount	53,600,000		59,000,000
Debt Instrument, Convertible, Remaining Discount Amortization Period	6 years 2 months 16 days
Debt Issuance Cost	11,700,000
Unamortized Debt Issuance Expense	9,100,000
Interest Expense, Debt	5,300,000
Debt Instrument, Frequency of Periodic Payment	semiannually
Long-term Debt	348,942,000
Long-term Debt, Fair Value	$ 467,400,000