Fair Value Measurements - Summary of Assets and (Liabilities) Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 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	$ 60,707	$ 56,556	$ 51,101
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation plan assets	100,365	84,435
Available-for-sale debt securities	60,707	56,556
Deferred compensation plan liabilities	(100,365)	(84,435)
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps	(427)	1,096
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent purchase price liabilities	$ (28,005)	$ (39,708)