Derivative Instruments (Details) - USD ($) $ in Millions		Sep. 30, 2015		Dec. 31, 2014	Dec. 31, 2006
Derivative [Line Items]
Total stockholders' equity		$ 3,860.3	[1]	$ 3,397.4
Designated as hedging instrument | Interest rate swap, Expired, 2006 secured railcar equipment notes
Derivative [Line Items]
Notional Amount		$ 200.0			$ 200.0
Interest Rate	[2]	4.87%
Liability		$ 0.0
Designated as hedging instrument | Interest rate swap, Expired, TRIP Holdings warehouse loan
Derivative [Line Items]
Notional Amount		$ 788.5
Interest Rate	[2]	3.60%
Liability		$ 0.0
Designated as hedging instrument | Interest rate swap, Open, TRIP Master Funding secured railcar equipment notes
Derivative [Line Items]
Notional Amount		$ 48.8
Interest Rate	[2]	2.62%
Liability		$ 2.2
AOCL – loss/ (income)
Derivative [Line Items]
Total stockholders' equity		(1.8)		$ (6.6)
AOCL – loss/ (income) | Designated as hedging instrument | Interest rate swap, Expired, 2006 secured railcar equipment notes
Derivative [Line Items]
Total stockholders' equity		(1.1)
AOCL – loss/ (income) | Designated as hedging instrument | Interest rate swap, Expired, TRIP Holdings warehouse loan
Derivative [Line Items]
Total stockholders' equity		8.5
AOCL – loss/ (income) | Designated as hedging instrument | Interest rate swap, Open, TRIP Master Funding secured railcar equipment notes
Derivative [Line Items]
Total stockholders' equity		0.9
Noncontrolling Interest | Designated as hedging instrument | Interest rate swap, Expired, 2006 secured railcar equipment notes
Derivative [Line Items]
Total stockholders' equity		0.0
Noncontrolling Interest | Designated as hedging instrument | Interest rate swap, Expired, TRIP Holdings warehouse loan
Derivative [Line Items]
Total stockholders' equity		11.4
Noncontrolling Interest | Designated as hedging instrument | Interest rate swap, Open, TRIP Master Funding secured railcar equipment notes
Derivative [Line Items]
Total stockholders' equity		$ 1.2

[1] (unaudited)

[2] Weighted average fixed interest rate