Earnings Per Share (Details) - shares shares in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive common stock instruments outstanding (shares)	8,109	7,352
Shares issuable upon the release of restricted stock awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive common stock instruments outstanding (shares)	16	24
Shares issuable upon exercise of stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive common stock instruments outstanding (shares)	8,093	7,328