LEASES - Lease Data (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Lease Costs
Operating lease expense	$ 21,562	$ 19,974
Short-term lease expense	1,601	2,572
Total lease expense	23,163	22,546
Other Lease Information
Operating cash flows from operating leases	21,114	19,112
Right-of-use lease assets obtained in exchange for new lease liabilities	$ 15,253	$ 60,281
Lease Term and Discount Rate
Weighted average remaining lease term (in years)	6 years 8 months 12 days
Weighted average discount rate (percent)	3.10%
Lessee, Lease, Description [Line Items]
Weighted average remaining lease term, excluding Chicago office space (in years)	4 years 7 months 6 days
Chicago Office Space
Lessee, Lease, Description [Line Items]
Lease term (in years)	15 years