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Debt - Senior Secured Credit Facility (Details) - Secured debt - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Senior Secured Credit Facility      
Debt Instrument [Line Items]      
Early maturity date description 90 years    
Maximum borrowing capacity $ 2,000,000,000.0 $ 2,000,000,000.0  
Current borrowing capacity 1,300,000,000 1,300,000,000  
Aggregate elected commitment 1,200,000,000.0 1,200,000,000.0  
Amount outstanding $ 170,000,000 $ 170,000,000  
Credit facility, interest rate at period end (as a percent) 3.44% 3.44%  
Senior Secured Credit Facility | Minimum      
Debt Instrument [Line Items]      
Line of credit facility, unused capacity, commitment fee percentage   0.375%  
Senior Secured Credit Facility | Maximum      
Debt Instrument [Line Items]      
Line of credit facility, unused capacity, commitment fee percentage   0.50%  
Letter of Credit      
Debt Instrument [Line Items]      
Maximum borrowing capacity $ 80,000,000.0 $ 80,000,000.0  
Letters of credit outstanding $ 0 $ 0 $ 0