LEASES - Lease Data (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2018
Lease Costs
Operating lease expense		$ 22,373	$ 16,905	$ 63,855	$ 50,684
Short-term lease expense		3,626	2,605	10,225	8,022
Total lease expense		$ 25,999	$ 19,510	74,080	58,706
Other Lease Information
Operating cash flows from operating leases				60,569	49,925
Right-of-use lease assets obtained in exchange for new lease liabilities				$ 88,857	$ 42,387
Lease Term and Discount Rate
Weighted average remaining lease term (in years)	6 years 10 months 24 days	6 years 10 months 24 days		6 years 10 months 24 days
Weighted average discount rate (percent)	3.20%	3.20%		3.20%
Lessee, Lease, Description [Line Items]
Weighted average remaining lease term, excluding Chicago office space (in years)	4 years 9 months 18 days
Chicago Office Space
Lessee, Lease, Description [Line Items]
Lease term (in years)						15 years