Derivative Instruments - Interest rate hedges (Details) - USD ($) $ in Millions	Dec. 31, 2018	May 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2006
Derivative [Line Items]
Total stockholders' equity	$ 2,562.0		$ 4,858.0	$ 4,311.1	$ 4,048.7
AOCL - Loss/(Income)
Derivative [Line Items]
Total stockholders' equity	(8.3)		$ 0.3	$ (0.1)
Designated as hedging instrument | Interest rate swap, Expired, 2006 secured railcar equipment notes
Derivative [Line Items]
Derivative, Notional Amount	$ 200.0					$ 200.0
Interest Rate	4.87%
Liability	$ 0.0
Designated as hedging instrument | Interest rate swap, Expired, 2006 secured railcar equipment notes | AOCL - Loss/(Income)
Derivative [Line Items]
Total stockholders' equity	(0.3)
Designated as hedging instrument | Interest rate swap, Expired, 2006 secured railcar equipment notes | Noncontrolling interest
Derivative [Line Items]
Total stockholders' equity	0.0
Designated as hedging instrument | Interest Rate Swap, Expired, 2018 Secured Railcar Equipment Notes [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 249.3
Designated as hedging instrument | Interest Rate Swap, Expired, 2018 Secured Railcar Equipment Notes [Domain]
Derivative [Line Items]
Derivative, Notional Amount		$ 1.4
Interest Rate	4.41%
Liability	$ 0.0
Designated as hedging instrument | Interest Rate Swap, Expired, 2018 Secured Railcar Equipment Notes [Domain] | AOCL - Loss/(Income)
Derivative [Line Items]
Total stockholders' equity	1.3
Designated as hedging instrument | Interest Rate Swap, Expired, 2018 Secured Railcar Equipment Notes [Domain] | Noncontrolling interest
Derivative [Line Items]
Total stockholders' equity	0.0
Designated as hedging instrument | Interest rate swap, Expired, TRIP Holdings warehouse loan
Derivative [Line Items]
Derivative, Notional Amount	$ 788.5
Interest Rate	3.60%
Liability	$ 0.0
Designated as hedging instrument | Interest rate swap, Expired, TRIP Holdings warehouse loan | AOCL - Loss/(Income)
Derivative [Line Items]
Total stockholders' equity	3.1
Designated as hedging instrument | Interest rate swap, Expired, TRIP Holdings warehouse loan | Noncontrolling interest
Derivative [Line Items]
Total stockholders' equity	4.1
Designated as hedging instrument | Interest Rate Swap, Open, TRIP Master Funding secured railcar equipment notes
Derivative [Line Items]
Derivative, Notional Amount	$ 34.8
Interest Rate	2.62%
Liability	$ 0.0
Designated as hedging instrument | Interest Rate Swap, Open, TRIP Master Funding secured railcar equipment notes | AOCL - Loss/(Income)
Derivative [Line Items]
Total stockholders' equity	0.2
Designated as hedging instrument | Interest Rate Swap, Open, TRIP Master Funding secured railcar equipment notes | Noncontrolling interest
Derivative [Line Items]
Total stockholders' equity	0.3
Designated as hedging instrument | 2017 Promissory note
Derivative [Line Items]
Derivative, Notional Amount	$ 169.3
Libor plus margin for 2017 promissory note	3.00%
Derivative asset	$ 0.0
Designated as hedging instrument | 2017 Promissory note | AOCL - Loss/(Income)
Derivative [Line Items]
Total stockholders' equity	(0.7)
Designated as hedging instrument | 2017 Promissory note | Noncontrolling interest
Derivative [Line Items]
Total stockholders' equity	0.0
Designated as hedging instrument | Interest Rate Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	396.1
Liability	$ (12.9)
Libor plus margin for 2017 promissory note	3.13%
Designated as hedging instrument | Interest Rate Swap [Member] | AOCL - Loss/(Income)
Derivative [Line Items]
Total stockholders' equity	$ 12.8
Designated as hedging instrument | Interest Rate Swap [Member] | Noncontrolling interest
Derivative [Line Items]
Total stockholders' equity	$ 0.0