Director and Employee Benefit Plans - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Duration of service for eligibility	3 months
Employer matching contribution, percent	3.00%
Maximum annual contribution per employee, percent	50.00%
Employer's maximum additional match, percentage	2.00%
Employer's discretionary matching contribution	4.00%
Cash contribution to profit-sharing plan	$ 3.0	$ 2.7	$ 2.5
Noncontributory Defined Benefit Multi-Employer Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Retirement plan expense	$ 0.7	$ 0.6	$ 0.6