Derivative Instruments - Interest rate hedges (Details) - USD ($) $ in Millions	Sep. 30, 2017		Dec. 31, 2016	Dec. 31, 2006
Derivative [Line Items]
Total stockholders' equity	$ 4,350.1	[1]	$ 4,311.1
AOCL – loss/ (income)
Derivative [Line Items]
Total stockholders' equity	0.3		$ (0.1)
Designated as hedging instrument | Interest rate swap, Expired, 2006 secured railcar equipment notes
Derivative [Line Items]
Notional Amount	$ 200.0			$ 200.0
Weighted average fixed interest rate	4.87%
Derivative liabilities	$ 0.0
Designated as hedging instrument | Interest rate swap, Expired, 2006 secured railcar equipment notes | AOCL – loss/ (income)
Derivative [Line Items]
Total stockholders' equity	(0.5)
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]
Notional Amount	$ 788.5
Weighted average fixed interest rate	3.60%
Derivative liabilities	$ 0.0
Designated as hedging instrument | Interest rate swap, Expired, TRIP Holdings warehouse loan | AOCL – loss/ (income)
Derivative [Line Items]
Total stockholders' equity	4.5
Designated as hedging instrument | Interest rate swap, Expired, TRIP Holdings warehouse loan | Noncontrolling Interest
Derivative [Line Items]
Total stockholders' equity	6.0
Designated as hedging instrument | Interest rate swap, Open, TRIP Master Funding secured railcar equipment notes
Derivative [Line Items]
Notional Amount	$ 34.8
Weighted average fixed interest rate	2.62%
Derivative liabilities	$ 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 | Interest rate cap, Open, 2017 promissory note
Derivative [Line Items]
Notional Amount	$ 178.4
Libor plus margin for 2017 promissory note	3.00%
Derivative asset	$ 1.8
Designated as hedging instrument | Interest rate cap, Open, 2017 promissory note | AOCL – loss/ (income)
Derivative [Line Items]
Total stockholders' equity	0.6
Designated as hedging instrument | Interest rate cap, Open, 2017 promissory note | Noncontrolling Interest
Derivative [Line Items]
Total stockholders' equity	$ 0.0

[1] (unaudited)