Fair Value Measurements - Marketable Securities and Deferred Compensation Fund Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Assets:
Marketable securities	$ 73,221	$ 67,730
Deferred compensation fund	28,885	24,119
Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	26,165	20,972
Significant Other Observable Inputs (Level 2)
Financial Assets:
Deferred compensation fund	2,720	3,147
Significant Unobservable Inputs (Level 3)
Financial Assets:
Deferred compensation fund	0	0
Carrying Amount
Financial Assets:
Deferred compensation fund	28,885	24,119
Total Fair Value
Financial Assets:
Deferred compensation fund	28,885	24,119
Municipal bonds — available-for-sale | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Marketable securities	0	0
Municipal bonds — available-for-sale | Significant Other Observable Inputs (Level 2)
Financial Assets:
Marketable securities	73,221	67,730
Municipal bonds — available-for-sale | Significant Unobservable Inputs (Level 3)
Financial Assets:
Marketable securities	0	0
Municipal bonds — available-for-sale | Carrying Amount
Financial Assets:
Marketable securities	73,221	67,730
Municipal bonds — available-for-sale | Total Fair Value
Financial Assets:
Marketable securities	73,221	67,730
Money Market | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	0	0
Money Market | Significant Other Observable Inputs (Level 2)
Financial Assets:
Deferred compensation fund	2,720	3,147
Money Market | Significant Unobservable Inputs (Level 3)
Financial Assets:
Deferred compensation fund	0	0
Money Market | Carrying Amount
Financial Assets:
Deferred compensation fund	2,720	3,147
Money Market | Total Fair Value
Financial Assets:
Deferred compensation fund	2,720	3,147
Balanced and Lifestyle | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	8,523	7,162
Balanced and Lifestyle | Significant Other Observable Inputs (Level 2)
Financial Assets:
Deferred compensation fund	0	0
Balanced and Lifestyle | Significant Unobservable Inputs (Level 3)
Financial Assets:
Deferred compensation fund	0	0
Balanced and Lifestyle | Carrying Amount
Financial Assets:
Deferred compensation fund	8,523	7,162
Balanced and Lifestyle | Total Fair Value
Financial Assets:
Deferred compensation fund	8,523	7,162
Large Cap Growth | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	7,802	5,583
Large Cap Growth | Significant Other Observable Inputs (Level 2)
Financial Assets:
Deferred compensation fund	0	0
Large Cap Growth | Significant Unobservable Inputs (Level 3)
Financial Assets:
Deferred compensation fund	0	0
Large Cap Growth | Carrying Amount
Financial Assets:
Deferred compensation fund	7,802	5,583
Large Cap Growth | Total Fair Value
Financial Assets:
Deferred compensation fund	7,802	5,583
Small Cap Value | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund		2,933
Small Cap Value | Significant Other Observable Inputs (Level 2)
Financial Assets:
Deferred compensation fund		0
Small Cap Value | Significant Unobservable Inputs (Level 3)
Financial Assets:
Deferred compensation fund		0
Small Cap Value | Carrying Amount
Financial Assets:
Deferred compensation fund		2,933
Small Cap Value | Total Fair Value
Financial Assets:
Deferred compensation fund		2,933
Small Cap Growth | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	3,442
Small Cap Growth | Significant Other Observable Inputs (Level 2)
Financial Assets:
Deferred compensation fund	0
Small Cap Growth | Significant Unobservable Inputs (Level 3)
Financial Assets:
Deferred compensation fund	0
Small Cap Growth | Carrying Amount
Financial Assets:
Deferred compensation fund	3,442
Small Cap Growth | Total Fair Value
Financial Assets:
Deferred compensation fund	3,442
Fixed Income | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	3,050	2,752
Fixed Income | Significant Other Observable Inputs (Level 2)
Financial Assets:
Deferred compensation fund	0	0
Fixed Income | Significant Unobservable Inputs (Level 3)
Financial Assets:
Deferred compensation fund	0	0
Fixed Income | Carrying Amount
Financial Assets:
Deferred compensation fund	3,050	2,752
Fixed Income | Total Fair Value
Financial Assets:
Deferred compensation fund	3,050	2,752
International | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	1,531	1,132
International | Significant Other Observable Inputs (Level 2)
Financial Assets:
Deferred compensation fund	0	0
International | Significant Unobservable Inputs (Level 3)
Financial Assets:
Deferred compensation fund	0	0
International | Carrying Amount
Financial Assets:
Deferred compensation fund	1,531	1,132
International | Total Fair Value
Financial Assets:
Deferred compensation fund	1,531	1,132
Mid Cap Growth | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	1,817	1,410
Mid Cap Growth | Significant Other Observable Inputs (Level 2)
Financial Assets:
Deferred compensation fund	0	0
Mid Cap Growth | Significant Unobservable Inputs (Level 3)
Financial Assets:
Deferred compensation fund	0	0
Mid Cap Growth | Carrying Amount
Financial Assets:
Deferred compensation fund	1,817	1,410
Mid Cap Growth | Total Fair Value
Financial Assets:
Deferred compensation fund	$ 1,817	$ 1,410