Employee Benefit Plans - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Current deferred compensation liability	$ 2,300,000	$ 2,900,000
Deferred compensation plan	31,800,000	28,900,000
Deferred compensation plan assets	33,400,000	31,400,000
Supplemental executive retirement plan	15,800,000	$ 14,400,000
Employer contributions to benefit plans	0
Expected funding of the SERP in the current year	$ 0