LEASES - Lease Data (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Lease Costs
Operating lease expense	$ 16,905	$ 50,684
Short-term lease expense	2,605	8,022
Total lease expense	$ 19,510	58,706
Other Lease Information
Operating cash flows from operating leases		49,925
Right-of-use lease assets obtained in exchange for new lease liabilities		$ 42,387
Lease Term and Discount Rate
Weighted average remaining lease term (in years)	7 years 9 months 18 days	7 years 9 months 18 days
Weighted average discount rate (percent)	3.60%	3.60%
Lessee, Lease, Description [Line Items]
Weighted average remaining lease term, excluding Chicago office space (in years)		4 years 6 months
Chicago Office Space
Lessee, Lease, Description [Line Items]
Lease term (in years)			15 years