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Fair Value Measurements - Summary of Assets and (Liabilities) Measured at Fair Value on a Recurring Basis (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Available-for-sale debt securities $ 60,822 $ 56,556 $ 51,101
Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Deferred compensation plan assets 100,529 84,435  
Available-for-sale debt securities 60,822 56,556  
Certified deposits and other depository assets 2,284 2,300  
Deferred compensation plan liabilities (100,529) (84,435)  
Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Interest rate swaps (688) 1,096  
Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Contingent purchase price liabilities $ (31,869) $ (39,708)