Line of Credit Payable (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Revolving credit facility borrowing capacity	$ 900,000,000
Revolving credit facility weighted average outstanding balance	$ 187,331,000
Revolving credit facility weighted average interest rate	2.70%
Option to increase facility size	$ 1,600,000,000
Line of credit payable	166,700,000	$ 120,500,000
Line of credit facility available for future borrowings	$ 733,300,000
Line of Credit | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.875%
Letter of Credit
Line of Credit Facility [Line Items]
Line of credit facility, undrawn letters of credit	$ 58,000