Deferred Compensation Benefits - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Compensation Text Details [Line Items]
Comprehensive pension gain (loss), net of tax	$ (606)	$ 1,365	$ (2,828)
Amounts not recognized as net periodic benefit, net of tax	(7,713)	(8,319)
Pension loss adjustment, net of tax	600	(1,400)	2,800
BPP [Member]
Deferred Compensation Text Details [Line Items]
Increase to noncurrent assets and offsetting adjustment to direct costs	300
Increase in income tax expense and deferred tax liabilities	100
Benefit obligation	$ 9,700	$ 8,400
Discount rate	1.62%	1.65%
Change in discount rate	0.03%
Targeted minimum return	3.25%	3.25%
Amounts not recognized as net periodic benefit, net of tax	$ 300	$ 0
NDBP [Member]
Deferred Compensation Text Details [Line Items]
Benefit obligation	$ 13,400	$ 13,300
Discount rate	1.80%	1.30%
Change in discount rate	0.50%
Increase (decrease) in plan liabilities	$ (1,400)
Targeted minimum return	4.00%	4.00%
Guaranteed return	4.00%	4.00%
Amounts not recognized as net periodic benefit, net of tax	$ 6,100	$ 6,900
ESBP [Member]
Deferred Compensation Text Details [Line Items]
Benefit obligation	6,400	6,800
Comprehensive pension gain (loss), net of tax	$ (400)	$ (400)
Discount rate	3.06%	3.41%
Change in discount rate	0.35%
Increase (decrease) in plan liabilities	$ 150
Estimated future employer contributions, next year	700
Amounts not recognized as net periodic benefit, net of tax	1,400	$ 1,400
ESBP, NDBP and BPP [Member]
Deferred Compensation Text Details [Line Items]
Benefit obligation	29,497	28,460	27,541
Pension loss adjustment, net of tax	100	(1,800)	2,300
Amount to be recognized as components of net periodic benefit in next year	300
Employer contributions	1,152	691
Key Employee Non Qualified Deferred Compensation Plan [Member]
Deferred Compensation Text Details [Line Items]
Estimated future employer contributions, next year	100
Employer contributions	100	200	100
Non Employee Director Deferred Compensation Plan [Member]
Deferred Compensation Text Details [Line Items]
Employer contributions	$ 500	$ 400	$ 500