Share-Based Compensation - Summary of Additional Stock Option Information (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Total intrinsic value of stock options outstanding	[1]	$ 2,473	$ 1,753	$ 740
Total intrinsic value of stock options exercised		0	0	1,474
Cash received from the exercise of stock options		0	0	898
Tax benefits realized for tax deductions related to stock option exercises		$ 0	$ 0	$ 104

[1] The intrinsic value of stock options outstanding represents the difference between our closing stock price on December 31 of each year and the exercise price, multiplied by the number of in-the-money stock options.