Long Term Debt (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Aug. 01, 2017 USD ($)	Jun. 10, 2014 USD ($)
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 1,052,500
Gain (Loss) on Extinguishment of Debt	0	$ (7,962)	$ 0
Debt Instrument, Unamortized Discount	(33,191)	(42,902)
Long-term Debt, Fair Value	1,100,000	1,200,000
Unamortized Debt Issuance Expense	(6,180)	(7,654)
Long-term debt	1,013,129	1,001,944
Interest Expense	63,500	59,200	42,700
Embedded Derivative, Gain (Loss) on Embedded Derivative, Net	0	(200)	(500)
Debt, Long-term and Short-term, Combined Amount	1,013,129	1,001,944
Long-term Debt, Current Maturities	0	0
Total long-term debt, less current portion	$ 1,013,129	1,001,944
6% Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Issuance Date	Jun. 27, 2017
Debt Instrument, Face Amount	$ 650,000	650,000
Debt Instrument, Interest Rate, Stated Percentage	6.00%
Proceeds from Debt, Net of Issuance Costs	$ 636,500
Debt Instrument, Frequency of Periodic Payment	semi-annually
Debt Instrument, Maturity Date	Jul. 15, 2025
Long-term Debt, Fair Value	$ 645,500	684,100
8% Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount				$ 250,000
Debt Instrument, Repurchase Amount				$ 265,000
Gain (Loss) on Extinguishment of Debt		8,000
Convertible Debt Securities [Member]
Debt Instrument [Line Items]
Debt Instrument, Issuance Date	Jun. 10, 2014
Debt Instrument, Face Amount	$ 402,500	402,500
Debt Instrument, Interest Rate, Stated Percentage	3.25%
Proceeds from Debt, Net of Issuance Costs	$ 391,400
Debt Instrument, Frequency of Periodic Payment	semiannually
Debt Instrument, Maturity Date	Jun. 15, 2029
Debt Instrument, Convertible, Conversion Ratio	14.7014
Debt Instrument, Convertible, Conversion Price | $ / shares	$ 68.02
Debt Instrument, Unamortized Discount	$ (23,534)	(32,189)			$ (59,000)
Debt Instrument, Convertible, Carrying Amount of Equity Component	$ 37,700	37,700			$ 37,700
Debt Instrument, Convertible, Remaining Discount Amortization Period	2 years 5 months 16 days
Long-term Debt, Fair Value	$ 457,000	504,500
Debt Instrument, Convertible, If-converted Value in Excess of Principal	8,000	38,100
Unamortized Debt Issuance Expense	(4,205)	(5,667)
Long-term debt	374,761	364,644
Interest Expense, Debt, Excluding Amortization	13,081	13,081	13,081
Amortization of Debt Discount (Premium)	8,655	8,167	7,707
Amortization of Debt Issuance Costs	1,462	1,335	1,217
Interest Expense	$ 23,198	$ 22,583	$ 22,005