LEASES - Lease Data (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Lease Costs
Operating Lease, Cost		$ 68,489
Short-term Lease, Cost		11,440
Total lease expense		79,929
Other Lease Information
Operating cash flows from operating leases		66,489
Right-of-use lease assets obtained in exchange for new lease liabilities		$ 101,966
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)	3.40%	3.40%
Weighted average remaining lease term, excluding Chicago office space (in years)		5 years 2 months 12 days
One leased location
Other Lease Information
Right-of-use lease assets obtained in exchange for new lease liabilities	$ 35,500
Chicago office space
Lease Term and Discount Rate
Lease term (in years)			15 years