Derivative Instruments - Interest rate hedges - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2006
Derivative [Line Items]
Total stockholders' equity	$ 4,858.0	$ 4,311.1	$ 4,048.7	$ 3,397.4
AOCL - Loss/(Income)
Derivative [Line Items]
Total stockholders' equity	0.3	$ (0.1)	$ (1.3)	$ (6.6)
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	(0.4)
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, 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	4.0
Designated as hedging instrument | Interest rate swap, Expired, TRIP Holdings warehouse loan | Noncontrolling interest
Derivative [Line Items]
Total stockholders' equity	5.4
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	0.3
Designated as hedging instrument | Interest Rate Swap, Open, TRIP Master Funding secured railcar equipment notes | Noncontrolling interest
Derivative [Line Items]
Total stockholders' equity	0.5
Designated as hedging instrument | 2017 Promissory note
Derivative [Line Items]
Derivative, Notional Amount	$ 176.1
Libor plus margin for 2017 promissory note	3.00%
Derivative asset	$ 1.6
Designated as hedging instrument | 2017 Promissory note | AOCL - Loss/(Income)
Derivative [Line Items]
Total stockholders' equity	0.8
Designated as hedging instrument | 2017 Promissory note | Noncontrolling interest
Derivative [Line Items]
Total stockholders' equity	$ 0.0