Share-Based Compensation - Employee Stock Purchase Plan (Details) shares in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant (in shares)	0.5
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)	4.1
Shares available for future grant (in shares)	2.2
Annual earnings withheld to purchase common stock | $	$ 21,250
Percent of IRS limitation	85.00%
Maximum fair value of common stock purchased | $	$ 25,000