Employee Benefit Plans (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) years period shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares
Employee Benefit [Line Items]
Number of Look-back Period Under Employee Stock Purchase Plan | period	2
Look-back Period Under Employee Stock Purchase Plan	6 months
Discount On Common Stock Under Employee Stock Purchase Plan, Percent	10.00%
Stock Issued During Period, Shares, Employee Stock Purchase Plans | shares	38,387	50,116	42,134
Stock Issued During Period, Value, Employee Stock Purchase Plan	$ 1,500	$ 1,800	$ 1,200
Shares Available For Purchase Under Employee Stock Purchase Plan | shares	411,107
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Balance at beginning of year	$ 26,263	25,544
Change in actuarial loss	(6,453)	62
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	148	0
Benefit expense	1,600	2,201
Balance at end of year	20,553	26,263	25,544
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
Defined Benefit Plan, Benefit Obligation, Benefits Paid	$ 1,005	1,544
Pension Plan [Member]
Employee Benefit [Line Items]
Deferred Compensation Arrangement with Individual, Requisite Service Period	1 month
Deferred Compensation Arrangement with Individual, Contribution To Plan By Employee, Percent Of Eligible Compensation	75.00%
Deferred Compensation Arrangement with Individual, Contribution Requirements, Matching Percent	50.00%
Deferred Compensation Arrangement with Individual, Contributions by Employer	$ 2,700	2,400	2,300
Pension Plan [Member] | Minimum [Member]
Employee Benefit [Line Items]
Deferred Compensation Arrangement with Individual, Eligible Age | years	18
Pension Plan [Member] | Maximum [Member]
Employee Benefit [Line Items]
Deferred Compensation Arrangement with Individual, Contribution Requirements, Percent Of Each Employee Compensation	3.00%
Deferred Profit Sharing [Member]
Employee Benefit [Line Items]
Deferred Compensation Arrangement with Individual, Requisite Service Period	1 month
Deferred Compensation Arrangement with Individual, Contribution To Plan By Employee, Percent Of Eligible Compensation	75.00%
Deferred Compensation Arrangement with Individual, Contribution Requirements, Matching Percent	50.00%
Deferred Compensation Arrangement with Individual, Contributions by Employer	$ 5,700	5,100	5,200
Deferred Profit Sharing [Member] | Minimum [Member]
Employee Benefit [Line Items]
Deferred Compensation Arrangement with Individual, Eligible Age | years	18
Deferred Profit Sharing [Member] | Maximum [Member]
Employee Benefit [Line Items]
Deferred Compensation Arrangement with Individual, Contribution Requirements, Percent Of Each Employee Compensation	3.00%
ESP Plan [Member]
Employee Benefit [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date	90.00%
Unit Plans [Member]
Employee Benefit [Line Items]
Deferred Compensation Arrangement with Individual, Vesting Period	10
Deferred Compensation Arrangement with Individual, Benefit Period	10
Deferred Compensation Arrangement with Individual, Recorded Liability	$ 4,600	4,800
Deferred Compensation Arrangement with Individual, Compensation Expense	$ 452	$ 467	$ 859
SERP [Member]
Employee Benefit [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	3.80%	4.09%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Normal Retirement Age | years	65
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Annual Cost Of Living Benefit Adjustment	2.00%
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2016	$ 936
2017	963
2018	1,971
2019	2,095
2020	1,135
2021 through 2025	7,563
Total	$ 14,663