Revenue and Contract Balances (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Total operating revenue	$ 77.8	$ 89.1	$ 232.5	$ 327.6
Commercial Real Estate
Disaggregation of Revenue [Line Items]
Commercial real estate	35.7	42.7	113.1	118.6
Land Operations
Disaggregation of Revenue [Line Items]
Revenues	7.7	8.5	29.0	82.4
Land Operations | Development sales revenue
Disaggregation of Revenue [Line Items]
Revenues	2.0	0.8	7.9	31.2
Land Operations | Unimproved/other property sales revenue
Disaggregation of Revenue [Line Items]
Revenues	0.0	1.5	3.7	32.4
Land Operations | Other operating revenue
Disaggregation of Revenue [Line Items]
Revenues	5.7	6.2	17.4	18.8
Materials & Construction
Disaggregation of Revenue [Line Items]
Revenues	$ 34.4	$ 37.9	$ 90.4	$ 126.6