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DEBT (Details) - USD ($)
$ in Thousands
1 Months Ended 3 Months Ended
Jun. 30, 2017
Oct. 31, 2016
Apr. 30, 2015
Mar. 31, 2018
Oct. 20, 2016
Apr. 01, 2015
Debt            
Line of Credit Facility, Expiration Date       Jun. 30, 2022    
Revolving credit agreement            
Debt            
Borrowing capacity under the line of credit       $ 400,000    
Line of Credit Facility, Initiation Date Jun. 30, 2017          
Covenant compliance description       As of March 31, 2018, the Company was in compliance with all of its covenants    
Additional increase in borrowing capacity of the line of credit available at the entity's option       $ 100,000    
Debt Instrument, Term       5 years    
Senior Notes [Member]            
Debt            
Debt, Weighted Average Interest Rate       3.30%    
Senior Notes 2015 [Member]            
Debt            
Debt Instrument, Initiation Date     Apr. 01, 2015      
Debt Instrument, Face Amount           $ 350,000
Debt, Weighted Average Interest Rate           3.50%
Debt Instrument, Covenant Compliance       As of March 31, 2018, the Company was in compliance with all of its debt covenants    
Weighted Average [Member]            
Debt            
Debt Instrument, Term   18 years 19 years 18 years    
Senior Notes 2016 [Member]            
Debt            
Debt Instrument, Initiation Date   Oct. 20, 2016        
Debt Instrument, Face Amount         $ 350,000  
Debt, Weighted Average Interest Rate         3.10%  
Debt Instrument, Covenant Compliance       As of March 31, 2018, the Company was in compliance with all of its debt covenants    
Minimum [Member] | Senior Notes 2015 [Member]            
Debt            
Debt Instrument, Term     10 years      
Minimum [Member] | Senior Notes 2016 [Member]            
Debt            
Debt Instrument, Term   12 years        
Maximum | Senior Notes 2015 [Member]            
Debt            
Debt Instrument, Term     30 years      
Maximum | Senior Notes 2016 [Member]            
Debt            
Debt Instrument, Term   25 years        
Senior Notes Series A [Member] | Senior Notes 2015 [Member]            
Debt            
Debt Instrument, Maturity Date       Aug. 20, 2025    
Debt Instrument, Interest Rate, Stated Percentage       3.15%    
Debt Instrument, Face Amount       $ 100,000    
Senior Notes Series A [Member] | Senior Notes 2016 [Member]            
Debt            
Debt Instrument, Maturity Date       Oct. 20, 2028    
Debt Instrument, Interest Rate, Stated Percentage       2.75%    
Debt Instrument, Face Amount       $ 100,000    
Senior Notes Series B [Member] | Senior Notes 2015 [Member]            
Debt            
Debt Instrument, Maturity Date       Aug. 20, 2030    
Debt Instrument, Interest Rate, Stated Percentage       3.35%    
Debt Instrument, Face Amount       $ 100,000    
Senior Notes Series B [Member] | Senior Notes 2016 [Member]            
Debt            
Debt Instrument, Maturity Date       Oct. 20, 2033    
Debt Instrument, Interest Rate, Stated Percentage       3.03%    
Debt Instrument, Face Amount       $ 100,000    
Senior Notes Series C [Member] | Senior Notes 2015 [Member]            
Debt            
Debt Instrument, Maturity Date       Apr. 01, 2035    
Debt Instrument, Interest Rate, Stated Percentage       3.61%    
Debt Instrument, Face Amount       $ 50,000    
Senior Notes Series C [Member] | Senior Notes 2016 [Member]            
Debt            
Debt Instrument, Maturity Date       Oct. 20, 2037    
Debt Instrument, Interest Rate, Stated Percentage       3.27%    
Debt Instrument, Face Amount       $ 100,000    
Senior Notes Series D [Member] | Senior Notes 2015 [Member]            
Debt            
Debt Instrument, Maturity Date       Apr. 01, 2045    
Debt Instrument, Interest Rate, Stated Percentage       4.02%    
Debt Instrument, Face Amount       $ 100,000    
Senior Notes Series D [Member] | Senior Notes 2016 [Member]            
Debt            
Debt Instrument, Maturity Date       Oct. 20, 2041    
Debt Instrument, Interest Rate, Stated Percentage       3.52%    
Debt Instrument, Face Amount       $ 50,000