Employee Benefit Plans (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Contribution	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Tax-exempt interest earned on the life insurance policies	$ 1,822	$ 1,864	$ 1,908
Executive Officers [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Employer contribution	4,300	3,800	3,500
Total accrued liability	43,300	37,400
Tax-exempt interest earned on the life insurance policies	1,800	1,900	1,900
Cash surrender value of life insurance policies	59,600	57,800
Senior Level Employees [Member] | Senior Management Retention Plan [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Employer contribution	765	627	530
Total accrued liability	$ 4,400	3,400
Profit Sharing Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined Contribution Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember
Minimum requisite service period	1 year
Number of annual employer contribution | Contribution	2
Employer discretionary contribution amount	$ 1,000	975	925
Employer mandatory contributions amount	$ 1,200	$ 1,200	$ 1,100
Annual vesting percentage, first year	0.00%
Annual vesting percentage, full year thereafter	25.00%
Benefit vesting period	5 years