Allowance for Credit Losses - Activity in the Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2020	Jun. 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		4.3	$ 1.6
Allowance for credit losses:
Allowance for Credit Loss	1.9	1.9
Provision for expected credit losses	(0.3)	0.4
Disposal of subsidiary		(0.1)
Allowance for Credit Loss	6.2	$ 5.7	$ 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	0.0	0.0
Provision for expected credit losses	0.0	0.0
Disposal of subsidiary		0.0
Financing Receivable - Allowance for Credit Loss, Ending Balance	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	1.6	1.6
Provision for expected credit losses	0.3	(0.3)
Disposal of subsidiary		0.0
Financing Receivable - Allowance for Credit Loss, Ending Balance	4.2	3.9	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	0.3	0.3
Provision for expected credit losses	0.0	(0.1)
Disposal of subsidiary		(0.1)
Contract Asset - Allowance for Credit Loss, Ending Balance	1.6	1.4	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