Earnings Per Share - Potentially Dilutive Shares Excluded From Diluted EPS Calculation (Details) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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	7,440	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)	12	2,403	1,057	2,403