Net Income (Loss) Per Share (Details) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	14,446,645	20,971,119	16,992,500	21,120,455
Stock appreciation rights
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	347	1,297,282	240,303	1,446,618
2019 Convertible Senior Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	4,358,629	5,567,112	5,178,146	5,567,112
2036 Convertible Senior Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	10,087,669	14,106,725	11,574,051	14,106,725