Stock-based Compensation Expense and Share Repurchase - Narrative (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) shares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of shares authorized to be repurchased | shares	500,000,000.0
Shares repurchased (shares) | shares	767,551
Share repurchased | $	$ 119,034
Shares repurchased, accrued liabilities | $	$ 4,000