Employee Benefit Plans (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)
Employee Benefit Plans
Percentage of contribution of covered employee's salaries or wages	2.50%
Contribution	$ 7,800	$ 7,800	$ 7,100
Amount payable to plan	$ 500	$ 200
Number of employees who are union members | item	6	5
Multiemployer Plans, Plan Contributions	$ 0	$ 0	$ 0
Period in which certain individuals are entitled to fixed annual payments	15 years
Maximum age under which certain individuals are entitled to fixed annual payments	65 years
Portion of benefits vesting after ten years of completed service (as a percent)	50.00%
Period of completed service over which 50% of benefits are vested	10 years
Portion of benefits vesting after fifteen years of completed service (as a percent)	75.00%
Period of completed service over which 75% of benefits are vested	15 years
Period of service over which benefits are fully vested	20 years
Unfunded benefit liability	$ 4,000	$ 4,000