Allowance for Credit Losses - Activity in the Allowance for Credit Losses (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($)
Financing Receivable - Allowance for Credit Loss [Roll Forward]
Financing Receivable - Allowance for Credit Loss, Beginning Balance	$ 1.6
Financing Receivable - Allowance for Credit Loss, Ending Balance	4.6
Allowance for credit losses:
Allowance for Credit Loss	1.9
Provision for expected credit losses	0.3
Allowance for Credit Loss	6.2
Impact of ASC 326 Adoption
Allowance for credit losses:
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
Provision for expected credit losses	0.0
Financing Receivable - Allowance for Credit Loss, Ending Balance	0.4
CRE | Impact of ASC 326 Adoption
Financing Receivable - Allowance for Credit Loss [Roll Forward]
Financing Receivable - Allowance for Credit Loss, Beginning Balance	0.4
Land Operations
Financing Receivable - Allowance for Credit Loss [Roll Forward]
Financing Receivable - Allowance for Credit Loss, Beginning Balance	1.6
Provision for expected credit losses	0.3
Financing Receivable - Allowance for Credit Loss, Ending Balance	4.2
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
Materials & Construction
Contract Asset - Allowance for Credit Loss [Roll Forward]
Contract Asset - Allowance for Credit Loss, Beginning Balance	0.3
Provision for expected credit losses	0.0
Contract Asset - Allowance for Credit Loss, Ending Balance	1.6
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