Share-Based Compensation - Employee Stock Purchase Plan (Details) - USD ($) shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant (in shares)	800
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Average weekly hours of work	20 hours
Service period	2 years
Stock options authorized to issue to employees (in shares)	4,100
Shares available for future grant (in shares)	2,300
Common shares purchased (in shares)	54	53	59
Annual earnings withheld to purchase common stock	$ 21,250
Percent of IRS limitation	85.00%
Maximum fair value of common stock purchased	$ 25,000