Earnings Per Share - Potentially Dilutive Shares Excluded From Diluted EPS Calculation (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Convertible Senior Notes Maturing May 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	8,997	8,997	8,997	8,997
Convertible Senior Notes Maturing September 2023
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	13,202	0	9,381	0
Convertible Senior Notes Maturing March 2032 (Redeemed May 2018)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	0	2,403	701	2,403