Share-Based Payments - Employee Stock Purchase Plan Narrative (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) hour $ / shares shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($) $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance (in shares)	2,424,652
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of hours worked (in hours) | hour	20
Number of months worked	5 months
Eligible purchase price of common stock under stock plan ( as a percent)	85.00%
Shares issued during period (in shares)	99,872
Shares issued under the plan (in shares)	842,238
Share available for grant (in shares)	1,582,414
Subscriptions outstanding (in shares)	53,855
Fair value of subscriptions outstanding (in dollars per share) | $ / shares	$ 4.83
Weighted average grant date fair value (in dollars per share) | $ / shares	$ 4.62	$ 4.32	$ 4.05
Cash received under plan | $	$ 1.2	$ 1.1	$ 1.0
Employee stock purchase plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payroll deduction range on cash compensation (as a percent)	15.00%