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Debt - Cabot Securitisation Senior Facility (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2017
GBP (£)
Aug. 23, 2017
USD ($)
Aug. 23, 2017
GBP (£)
Dec. 31, 2016
USD ($)
Debt Instrument [Line Items]          
Assets $ 4,490,712,000       $ 3,670,497,000
Cabot securitisation senior facility          
Debt Instrument [Line Items]          
Weighted average interest rate 3.10% 3.10%      
Revolving Credit Facility          
Debt Instrument [Line Items]          
Revolving credit facility $ 794,600,000.0        
Additional line of revolving credit facility $ 250,000,000        
Revolving Credit Facility | Cabot securitisation senior facility          
Debt Instrument [Line Items]          
Revolving credit facility   £ 300,000,000.0 $ 332,900,000 £ 260,000,000.0  
Additional line of revolving credit facility | £   40,000,000      
Amount drawn on revolving credit facility | £   290,000,000      
Revolving Credit Facility | Cabot securitisation senior facility | LIBOR          
Debt Instrument [Line Items]          
Basis spread on variable rate 2.85%        
Cabot securitisation senior facility          
Debt Instrument [Line Items]          
Outstanding borrowings for credit facility $ 391,790,000       $ 0
Collateral Pledged | Cabot securitisation senior facility          
Debt Instrument [Line Items]          
Assets $ 416,800,000 £ 308,500,000