LEASES - Lease Data (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Lease Costs
Operating lease expense	$ 21,220	$ 22,373	$ 64,241	$ 63,855
Short-term lease expense	2,871	3,626	5,934	10,225
Total lease expense	$ 24,091	$ 25,999	70,175	74,080
Other Lease Information
Operating cash flows from operating leases			70,997	60,569
Right-of-use lease assets obtained in exchange for new lease liabilities			$ 38,028	$ 88,857
Lease Term and Discount Rate
Weighted average remaining lease term (in years)	6 years 6 months		6 years 6 months
Weighted average discount rate (percent)	3.00%		3.00%
Lessee, Lease, Description [Line Items]
Weighted average remaining lease term, excluding Chicago office space (in years)			4 years 4 months 24 days
Chicago Office Space
Lessee, Lease, Description [Line Items]
Lease term (in years)	15 years		15 years