Debt - Unsecured Notes Payable (Details) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Total indebtedness	$ 1,543,753,000	$ 1,543,753,000	$ 1,488,718,000
Fund IV | Line of Credit | Unsecured Debt
Debt Instrument [Line Items]
Maximum borrowing capacity	21,500,000.0	21,500,000.0	100,000,000.0
Repayments of debt		74,100,000
Proceeds from (repayments of) lines of credit		10,000,000
Long-term line of credit, noncurrent	20,400,000	20,400,000	94,500,000
Remaining borrowing capacity	0	0	$ 55,500,000
Fund V | Subscription Line | Unsecured Debt
Debt Instrument [Line Items]
Maximum borrowing capacity	150,000,000.0	150,000,000.0
Repayments of debt	1,000,000
Long-term line of credit, noncurrent	44,400,000	44,400,000
Remaining borrowing capacity	105,600,000	$ 105,600,000
Proceeds from debt	$ 45,400,000