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Debt - Cabot Senior Revolving Credit Facility (Details)
12 Months Ended
Dec. 20, 2016
USD ($)
Oct. 06, 2016
Oct. 05, 2016
Nov. 11, 2015
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Nov. 11, 2015
GBP (£)
Sep. 20, 2012
USD ($)
Sep. 20, 2012
GBP (£)
Debt Instrument [Line Items]                  
Weighted average interest rate         3.56% 3.17%      
Cabot Credit Agreement [Member]                  
Debt Instrument [Line Items]                  
Credit facility               $ 82,700,000 £ 50,000,000.0
Revolving Credit Facility [Member]                  
Debt Instrument [Line Items]                  
Credit facility $ 781,700,000.0                
Cabot Credit Facility [Member]                  
Debt Instrument [Line Items]                  
Additional borrowing capacity, accordion provision | £             £ 50,000,000    
Maximum loan to value ratio       0.75%          
Outstanding borrowings for credit facility         $ 33,218,000 $ 54,089,000      
Weighted average interest rate         3.95% 3.86%      
Senior Loans [Member] | Cabot Credit Facility [Member]                  
Debt Instrument [Line Items]                  
Maximum loan to value ratio       25.00%          
LIBOR [Member] | Cabot Credit Facility [Member]                  
Debt Instrument [Line Items]                  
Debt instrument, initial spread percentage       3.25%          
LIBOR [Member] | Minimum [Member]                  
Debt Instrument [Line Items]                  
Debt instrument, initial spread percentage 2.50%                
Cabot Credit Agreement [Member] | Secured Debt [Member]                  
Debt Instrument [Line Items]                  
Credit facility       $ 316,200,000     £ 250,000,000    
Cabot Credit Agreement [Member] | Senior Loans [Member] | Revolving Credit Facility [Member]                  
Debt Instrument [Line Items]                  
Maximum utilization of credit facility       20.00%          
Cabot Credit Agreement [Member] | LIBOR [Member] | Secured Debt [Member] | Revolving Credit Facility [Member]                  
Debt Instrument [Line Items]                  
Debt instrument, initial spread percentage   3.25% 3.50%