Real Estate Joint Ventures - Narrative (Details) $ in Millions	1 Months Ended		3 Months Ended	9 Months Ended			12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016 USD ($)	Dec. 31, 2017	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2017	Apr. 30, 2017 apartment
Pier Park Crossings JV | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Number of units | apartment								240
Variable interest entity, ownership percentage			75.00%	65.00%
Windmark | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Variable interest entity, ownership percentage	49.00%	49.00%
Pier Park North | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Variable interest entity, ownership percentage	60.00%	60.00%
Artisan Park, L.L.C | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Variable interest entity, ownership percentage							74.00%
ALP Liquidating Trust
Equity Method Investment, Summarized Financial Information [Abstract]
Assets		$ 11.5		$ 10.8
Liabilities		0.6		0.5
Equity		$ 10.9		10.3
Net loss				$ 0.6	$ 0.8	$ 2.7