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Subsequent events - Narrative (Details) - Line of Credit - Senior Secured Credit Facility
Apr. 19, 2018
USD ($)
May 01, 2018
USD ($)
Apr. 18, 2018
USD ($)
Apr. 10, 2018
USD ($)
Mar. 31, 2018
USD ($)
Subsequent Event [Line Items]          
Current borrowing capacity         $ 1,000,000,000.0
Aggregate elected commitment         1,000,000,000.0
Letter of credit, maximum exposure         20,000,000
Amount borrowed         $ 55,000,000
Subsequent Event          
Subsequent Event [Line Items]          
Current borrowing capacity $ 1,300,000,000        
Aggregate elected commitment $ 1,200,000,000        
Early maturity period 90 days        
Reduction in interest rate 0.75%        
Letter of credit, maximum exposure $ 80,000,000        
Line of credit facility, unused capacity, commitment fee percentage 20.00%        
Total leverage ratio 3.00        
Amount borrowed   $ 110,000,000 $ 20,000,000 $ 35,000,000