Summary of Significant Accounting Policies - Remaining performance obligation (Details) $ in Millions	Mar. 31, 2018 USD ($)
Rail Group | Railcars
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 2,140.3
Revenue, remaining performance obligation expected to be delivered in current year	54.00%
Rail Group | Railcars | External Customers
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 1,420.7
Rail Group | Railcars | Leasing
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	719.6
Rail Group | Components and maintenance services
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 63.8
Revenue, remaining performance obligation expected to be delivered in current year	100.00%
Inland Barge Group
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 124.5
Revenue, remaining performance obligation expected to be delivered in current year	64.00%
Energy Equipment Group | Wind towers and utility structures
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 809.7
Revenue, remaining performance obligation expected to be delivered in current year	41.00%
Energy Equipment Group | Other
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 42.9
Revenue, remaining performance obligation expected to be delivered in current year	100.00%
Railcar Leasing and Management Services Group
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 127.1
Revenue, remaining performance obligation expected to be delivered in current year	15.00%