Organization and Basis of Presentations - Reconciliation of the Effects of the Revision (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Other liabilities	$ 9,273	$ 9,893
Total liabilities	1,232,238	1,172,900
Accumulated distributions in excess of net income	(394,516)	(384,934)	$ (368,769)	$ (361,547)
Noncontrolling interest	20,789	20,897	21,982	25,790
Total shareholder's equity	$ 880,813	888,598
As reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Other liabilities		6,800
Total liabilities		1,169,807
Accumulated distributions in excess of net income		(381,912)	(365,747)	(358,525)
Noncontrolling interest		20,968	22,053	25,861
Total shareholder's equity		891,691
Revision Of Certain Special Assessment Obligations On Undeveloped Land | Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Other liabilities		3,093
Total liabilities		3,093
Accumulated distributions in excess of net income		(3,022)	(3,022)	(3,022)
Noncontrolling interest		(71)	$ (71)	$ (71)
Total shareholder's equity		$ (3,093)