Employee Benefit Plans - Additional Information (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Current deferred compensation liability	$ 2,000,000	$ 2,900,000
Deferred compensation plan	30,000,000	28,900,000
Deferred compensation plan assets	32,000,000	31,400,000
Supplemental executive retirement plan	15,300,000	$ 14,400,000
Employer contributions to benefit plans	0
Expected funding of the SERP in the current year	$ 0