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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Share-based Compensation (Details)
12 Months Ended
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Period from grant date to retirement eligibility, threshold basis for amortization period of share based compensation cost 6 months
Maximum [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Non-employee director award, maximum service period (in years) 1 year
Minimum [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Requisite service period 6 months
Restricted Shares And Restricted Share Units [Member] | Maximum [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Requisite service period 36 months
Performance-based awards [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years
Performance-based awards [Member] | Maximum [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 36 months