Fair Value Measurements - Marketable Securities and Deferred Compensation Fund Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financial Assets:
Municipal bonds — available-for-sale	$ 90,711	$ 76,362	$ 73,221
Deferred compensation fund	37,247	29,113
Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	34,622	26,584
Significant Other Observable Inputs (Level 2)
Financial Assets:
Deferred compensation fund	2,625	2,529
Significant Unobservable Inputs (Level 3)
Financial Assets:
Deferred compensation fund	0	0
Carrying Amount
Financial Assets:
Deferred compensation fund	37,247	29,113
Municipal bonds — available-for-sale
Financial Assets:
Municipal bonds — available-for-sale	90,711	76,362	$ 73,221
Municipal bonds — available-for-sale | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Municipal bonds — available-for-sale	0	0
Municipal bonds — available-for-sale | Significant Other Observable Inputs (Level 2)
Financial Assets:
Municipal bonds — available-for-sale	90,711	76,362
Municipal bonds — available-for-sale | Significant Unobservable Inputs (Level 3)
Financial Assets:
Municipal bonds — available-for-sale	0	0
Municipal bonds — available-for-sale | Carrying Amount
Financial Assets:
Municipal bonds — available-for-sale	90,711	76,362
Money Market
Financial Assets:
Deferred compensation fund	2,625	2,529
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	2,625	2,529
Money Market | Significant Unobservable Inputs (Level 3)
Financial Assets:
Deferred compensation fund	0	0
Money Market | Carrying Amount
Financial Assets:
Deferred compensation fund	2,625	2,529
Balanced and Lifestyle
Financial Assets:
Deferred compensation fund	10,294	8,265
Balanced and Lifestyle | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	10,294	8,265
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	10,294	8,265
Large Cap Growth
Financial Assets:
Deferred compensation fund	11,369	8,195
Large Cap Growth | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	11,369	8,195
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	11,369	8,195
Small Cap Growth
Financial Assets:
Deferred compensation fund	4,120
Small Cap Growth | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	4,120
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	4,120
Small Cap Value
Financial Assets:
Deferred compensation fund		3,217
Small Cap Value | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund		3,217
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		3,217
Fixed Income
Financial Assets:
Deferred compensation fund	4,072	3,432
Fixed Income | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	4,072	3,432
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	4,072	3,432
International
Financial Assets:
Deferred compensation fund	1,932	1,485
International | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	1,932	1,485
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	1,932	1,485
Mid Cap Growth
Financial Assets:
Deferred compensation fund	2,835	1,990
Mid Cap Growth | Quoted Prices in Active Markets (Level 1)
Financial Assets:
Deferred compensation fund	2,835	1,990
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	$ 2,835	$ 1,990