Annual report pursuant to Section 13 and 15(d)

Borrowings (Credit Facility) (Narrative) (Details)

v3.8.0.1
Borrowings (Credit Facility) (Narrative) (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
Nov. 07, 2017
USD ($)
May 31, 2017
USD ($)
lender
Dec. 31, 2016
USD ($)
Line of Credit Facility [Line Items]        
Senior secured revolving credit facility $ 25,000,000     $ 0
Debt instrument outstanding $ 625,000,000     400,000,000
Senior Secured Revolving Credit Facility        
Line of Credit Facility [Line Items]        
Debt instrument maturity date May 25, 2022      
Institutional lenders | lender     17  
Maximum borrowing capacity   $ 700,000,000 $ 2,000,000,000  
Increase limit $ 650,000,000      
Current borrowing capacity   $ 500,000,000    
Debt instrument outstanding $ 25,000,000     $ 0
Interest rate at period end (as a percent) 3.11%      
Unused capacity, commitment fee (as a percent) 0.375%      
Senior Secured Revolving Credit Facility | LIBOR [Member] | Minimum        
Line of Credit Facility [Line Items]        
Debt instrument interest rate 2.00%      
Senior Secured Revolving Credit Facility | LIBOR [Member] | Maximum        
Line of Credit Facility [Line Items]        
Debt instrument interest rate 3.00%