Debt - Unsecured Notes Payable (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Total indebtedness	$ 1,602,847,000	$ 1,488,718,000
Fund IV | Line of Credit | Unsecured Debt
Debt Instrument [Line Items]
Maximum borrowing capacity	21,500,000.0	100,000,000.0
Long-term line of credit, noncurrent	20,400,000	94,500,000
Remaining borrowing capacity	1,100,000	5,500,000
Fund IV | Bridge facility | Unsecured Debt
Debt Instrument [Line Items]
Maximum borrowing capacity	50,000,000.0
Long-term line of credit, noncurrent	34,500,000	40,100,000
Remaining borrowing capacity	0	$ 0
Fund V | Subscription Line | Unsecured Debt
Debt Instrument [Line Items]
Maximum borrowing capacity	150,000,000.0
Long-term line of credit, noncurrent	113,200,000
Remaining borrowing capacity	36,800,000
Fund II | Loans | Unsecured Debt
Debt Instrument [Line Items]
Long-term line of credit, noncurrent	31,500,000
Remaining borrowing capacity	8,500,000
Fund II | Loans Maturing In September 2020 | Unsecured Debt
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 40,000,000.0
LIBOR | Fund V | Subscription Line | Unsecured Debt
Debt Instrument [Line Items]
Basis spread on variable rate	1.60%	1.60%
LIBOR | Fund II | Loans Maturing In September 2020 | Unsecured Debt
Debt Instrument [Line Items]
Basis spread on variable rate	1.40%