Allowance for Credit Losses - Activity in the Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended	12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable - Allowance for Credit Loss [Roll Forward]
Financing Receivable - Allowance for Credit Loss, Beginning Balance			$ 1.6	$ 1.6
Financing Receivable - Allowance for Credit Loss, Ending Balance	$ 4.5			4.5	$ 1.6
Allowance for credit losses:
Allowance for Credit Loss	5.7	$ 6.2	1.9	1.9
Provision for expected credit losses	(0.1)	0.4	(0.3)
Disposal of subsidiary		(0.1)
Allowance for Credit Loss	5.8	5.7	6.2	$ 5.8	$ 1.9
Accounting Standards Update [Extensible List]				us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member
Impact of ASC 326 Adoption
Allowance for credit losses:
Allowance for Credit Loss			4.0	$ 4.0
Allowance for Credit Loss					$ 4.0
CRE
Financing Receivable - Allowance for Credit Loss [Roll Forward]
Financing Receivable - Allowance for Credit Loss, Beginning Balance	0.4	0.4	0.0	0.0
Provision for expected credit losses	0.0	0.0	0.0
Disposal of subsidiary		0.0
Financing Receivable - Allowance for Credit Loss, Ending Balance	0.4	0.4	0.4	0.4	0.0
CRE | Impact of ASC 326 Adoption
Financing Receivable - Allowance for Credit Loss [Roll Forward]
Financing Receivable - Allowance for Credit Loss, Beginning Balance			0.4	0.4
Financing Receivable - Allowance for Credit Loss, Ending Balance					0.4
Land Operations
Financing Receivable - Allowance for Credit Loss [Roll Forward]
Financing Receivable - Allowance for Credit Loss, Beginning Balance	3.9	4.2	1.6	1.6
Provision for expected credit losses	0.2	(0.3)	0.3
Disposal of subsidiary		0.0
Financing Receivable - Allowance for Credit Loss, Ending Balance	4.1	3.9	4.2	4.1	1.6
Land Operations | Impact of ASC 326 Adoption
Financing Receivable - Allowance for Credit Loss [Roll Forward]
Financing Receivable - Allowance for Credit Loss, Beginning Balance			2.3	2.3
Financing Receivable - Allowance for Credit Loss, Ending Balance					2.3
Materials & Construction
Contract Asset - Allowance for Credit Loss [Roll Forward]
Contract Asset - Allowance for Credit Loss, Beginning Balance	1.4	1.6	0.3	0.3
Provision for expected credit losses	(0.1)	(0.1)	0.0
Disposal of subsidiary		(0.1)
Contract Asset - Allowance for Credit Loss, Ending Balance	$ 1.3	$ 1.4	1.6	1.3	0.3
Materials & Construction | Impact of ASC 326 Adoption
Contract Asset - Allowance for Credit Loss [Roll Forward]
Contract Asset - Allowance for Credit Loss, Beginning Balance			$ 1.3	$ 1.3
Contract Asset - Allowance for Credit Loss, Ending Balance					$ 1.3