Credit Facility (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Credit facility      
Outstanding balance $ 370,000,000   $ 223,000,000
Credit Facility      
Credit facility      
Maximum borrowing capacity 1,500,000,000    
Term of extension option 1 year    
Variable reference rate LIBOR    
Variable interest rate, basis points spread over variable reference rate (as a percent) 1.075%    
Line of credit facility, commitment fee basis points (as a percent) 0.175%    
Line of credit facility, all-in drawn variable interest rate (as a percent) 1.25%    
Unamortized debt issuance costs 3,400,000    
Current borrowing capacity available $ 1,130,000,000    
Average borrowing rate during the period (as a percent) 1.20% 1.20%  
Effective interest rate (as a percent) 1.20%