Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation plan assets	$ 136,059	$ 127,332
Available-for-sale debt securities	28,455	25,708	$ 60,659
Deferred compensation plan liabilities	(136,059)	(127,332)
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation plan assets	136,059	127,332
Available-for-sale debt securities	28,455	25,708
Other depository assets	4,834	0
Deferred compensation plan liabilities	(136,059)	(127,332)
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps	(786)	(2,565)
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent purchase price liabilities	$ (84,628)	$ (54,391)