Related-party transactions (Details)			3 Months Ended		6 Months Ended
	Apr. 01, 2015 extension	Mar. 31, 2015 property	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Related party transaction [Line Items]
Related party payables			$ 4,933,000		$ 4,933,000		$ 5,266,000
Affiliated entity | AAMC
Related party transaction [Line Items]
Base management fee, percent of qualified average invested capital		1.50%
Incentive management fee, return on invested capital					1.75%
Conversion fee, percent of market value of new rental properties		1.50%
Incentive management fee, percent of incentive fee payable in common stock					25.00%
Period in fiscal years return on invested equity capital evaluated for termination of agreement		2 years
Period of time required return rate evaluated per new agreement					21 months
Deficit of return on invested capital					60.40%
Affiliated entity | AAMC | Minimum
Related party transaction [Line Items]
Incentive management fee, return on invested capital		7.00%
Affiliated entity | AAMC | Maximum
Related party transaction [Line Items]
Incentive management fee, return on invested capital		8.25%
Affiliated entity | Asset management fee, threshold one | AAMC
Related party transaction [Line Items]
Incentive management fee, percent of average invested capital		25.00%
Incentive management fee, percent of invested capital in excess of threshold		20.00%
Affiliated entity | Asset management fee, threshold two | AAMC
Related party transaction [Line Items]
Incentive management fee, percent of average invested capital		1.75%
Base management fee, number of rental properties floor | property		2,500
Incentive management fee, number of rental properties cap | property		4,499
Incentive management fee, number of rental properties, floor | property		2,500
Incentive management fee, percent of invested capital in excess of threshold		22.50%
Affiliated entity | Asset management fee, threshold three | AAMC
Related party transaction [Line Items]
Incentive management fee, percent of average invested capital		2.00%
Incentive management fee, number of rental properties, floor | property		4,500
Incentive management fee, percent of invested capital in excess of threshold		25.00%
Affiliated entity | Asset Management Agreement | AAMC
Related party transaction [Line Items]
Contract term	15 years
Number of potential renewal extensions | extension	2
Automatic renewal term	5 years
Affiliated entity | Base management fee | AAMC
Related party transaction [Line Items]
Related party expenses			3,999,000	$ 4,506,000	$ 8,210,000	$ 8,630,000
Affiliated entity | Conversion fee | AAMC
Related party transaction [Line Items]
Related party expenses			434,000	544,000	1,038,000	946,000
Affiliated entity | Management incentive fee | AAMC
Related party transaction [Line Items]
Related party payables			0		0
Affiliated entity | Expense reimbursements | AAMC
Related party transaction [Line Items]
Related party expenses			$ 210,000	$ 357,000	$ 406,000	$ 357,000