Share-based Payments - Shares Reserved for Future Issuance (Details) - shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock options outstanding (in shares)	1,381,078	862,977	204,968	241,766
Possible future issuances under director compensation plan (in shares)	1,290,603
Common stock, reserved for future issuance (in shares)	2,671,681
Common stock, shares available to be issued under charter (in shares)	146,552,050