Summary of Significant Accounting Policies - Remaining performance obligation (Details) $ in Millions	Jun. 30, 2018 USD ($)
Rail Group | Railcars
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 2,664.3
Revenue, remaining performance obligation expected to be delivered in current year	34.00%
Rail Group | Railcars | External Customers
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 1,922.8
Rail Group | Railcars | Leasing
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	741.5
Rail Group | Components and maintenance services
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 66.4
Revenue, remaining performance obligation expected to be delivered in current year	63.00%
Inland Barge Group
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 198.4
Revenue, remaining performance obligation expected to be delivered in current year	44.00%
Energy Equipment Group | Wind towers and utility structures
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 780.1
Revenue, remaining performance obligation expected to be delivered in current year	37.00%
Energy Equipment Group | Other
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 53.2
Revenue, remaining performance obligation expected to be delivered in current year	86.00%
Railcar Leasing and Management Services Group
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 124.7
Revenue, remaining performance obligation expected to be delivered in current year	10.00%