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Debt - Cabot Senior Revolving Credit Facility (Details)
3 Months Ended 6 Months Ended
Oct. 06, 2016
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2016
Jun. 30, 2017
USD ($)
Jun. 30, 2016
Jun. 30, 2017
GBP (£)
Jun. 29, 2017
USD ($)
Dec. 31, 2016
USD ($)
Oct. 06, 2016
GBP (£)
Sep. 20, 2012
USD ($)
Sep. 20, 2012
GBP (£)
Debt Instrument [Line Items]                      
Credit facility, outstanding amount | $   $ 539,000,000   $ 539,000,000       $ 578,000,000      
Weighted average interest rate (percent)   4.06% 3.50% 3.90% 3.49%            
Cabot Credit Agreement                      
Debt Instrument [Line Items]                      
Revolving credit facility                   $ 82,700,000 £ 50,000,000.0
Revolving Credit Facility                      
Debt Instrument [Line Items]                      
Revolving credit facility | $             $ 801,700,000.0        
Revolving Credit Facility | Secured Debt | Cabot Credit Agreement                      
Debt Instrument [Line Items]                      
Revolving credit facility $ 316,200,000               £ 250,000,000    
Line of credit facility, increase to maximum borrowing capacity, accordion provision | £                 £ 50,000,000    
Loan-to-value credit facility utilization trigger (more than) 20.00%                    
Revolving Credit Facility | Secured Debt | Cabot Credit Agreement | LIBOR                      
Debt Instrument [Line Items]                      
Basis spread on variable rate 3.25%                    
Revolving Credit Facility | Secured Debt | Cabot Credit Agreement, Tranche with Expiration in September 2019                      
Debt Instrument [Line Items]                      
Revolving credit facility | £           £ 200,000,000.0          
Revolving Credit Facility | Secured Debt | Cabot Credit Agreement, Tranche with Expiration in March 2022                      
Debt Instrument [Line Items]                      
Revolving credit facility | £           £ 50,000,000.0          
Revolving Credit Facility | Senior Loans | Cabot Credit Agreement                      
Debt Instrument [Line Items]                      
Loan-to-value credit facility utilization trigger (more than) 20.00%                    
Cabot Credit Facility                      
Debt Instrument [Line Items]                      
Maximum loan to value ratio       75.00%              
Credit facility, outstanding amount | $   $ 81,188,000   $ 81,188,000       $ 33,218,000      
Weighted average interest rate (percent)   3.50% 4.01% 3.51% 4.01%            
Cabot Credit Facility | Senior Loans                      
Debt Instrument [Line Items]                      
Maximum loan to value ratio       25.00%