Line of Credit Payable (Details)	9 Months Ended
	Sep. 30, 2017 USD ($)
Line of Credit Facility [Line Items]
Revolving credit facility borrowing capacity	$ 650,000,000
Revolving credit facility weighted average outstanding balance	$ 100,545,000
Revolving credit facility weighted average interest rate	2.10%
Option to increase facility size	$ 1,000,000,000
Line of credit payable	0
Line of credit facility available for future borrowings	650,000,000
Letter of Credit
Line of Credit Facility [Line Items]
Line of credit facility, undrawn letters of credit	$ 230,000
Line of Credit | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.925%