Employee Benefit Plans - Stock Purchase Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee benefit plans [Line Items]
Document Fiscal Year Focus	2017
Employee Stock [Member] | Employee Stock Purchase Plan [Member]
Employee benefit plans [Line Items]
Proceeds from purchase of shares by employees	$ 1.0	$ 0.9	$ 0.8
Number of shares issued by the Company	33,666	39,055	38,565
Recorded in share-based compensation, net of deferral	$ 0.3	$ 0.2	$ 0.2