Summary of Significant Accounting Policies - Remaining performance obligation (Details) $ in Millions	Sep. 30, 2018 USD ($)
Rail Group | Railcars
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 3,200.8
Revenue, remaining performance obligation expected to be delivered in current year	20.00%
Rail Group | Railcars | External Customers
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 2,001.2
Rail Group | Railcars | Leasing
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	1,199.6
Rail Group | Components and maintenance services
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 58.5
Revenue, remaining performance obligation expected to be delivered in current year	36.00%
Inland Barge Group
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 210.4
Revenue, remaining performance obligation expected to be delivered in current year	21.00%
Energy Equipment Group | Wind towers and utility structures
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 700.3
Revenue, remaining performance obligation expected to be delivered in current year	23.00%
Energy Equipment Group | Other
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 83.5
Revenue, remaining performance obligation expected to be delivered in current year	37.00%
Railcar Leasing and Management Services Group
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 123.1
Revenue, remaining performance obligation expected to be delivered in current year	5.00%