LEASES - Lease Data (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2018
Lease Costs
Operating lease expense	$ 21,508	$ 16,957	$ 41,482	$ 33,779
Short-term lease expense	4,027	3,076	6,599	5,417
Total lease expense	$ 25,535	$ 20,033	48,081	39,196
Other Lease Information
Operating cash flows from operating leases			38,655	33,376
Right-of-use lease assets obtained in exchange for new lease liabilities			$ 70,993	$ 26,198
Lease Term and Discount Rate
Weighted average remaining lease term (in years)	7 years 1 month 6 days		7 years 1 month 6 days
Weighted average discount rate (percent)	3.30%		3.30%
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