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LEASES - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Jan. 01, 2019
Dec. 31, 2018
Lessee, Lease, Description [Line Items]      
Right-of-use lease assets $ 257,034    
Lease liabilities $ 264,738    
Weighted average remaining lease term, excluding Chicago office space (in years) 4 years 1 month 6 days    
ASU No. 2016-02      
Lessee, Lease, Description [Line Items]      
Right-of-use lease assets   $ 265,400  
Lease liabilities   $ 273,300  
Chicago Office Space      
Lessee, Lease, Description [Line Items]      
Lease term (in years)     15 years