Derivative Instruments - Interest rate hedges (Details) - USD ($) $ in Millions	Jun. 30, 2018		May 31, 2018	Dec. 31, 2017	Dec. 31, 2006
Derivative [Line Items]
Total stockholders' equity	$ 4,667.8	[1]		$ 4,858.0
AOCL – loss/ (income)
Derivative [Line Items]
Total stockholders' equity	0.8			$ 0.3
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	(0.3)
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
Derivative [Line Items]
Notional Amount	$ 249.3		$ 1.4
Weighted average fixed interest rate	4.41%
Derivative liabilities	$ 0.0
Designated as hedging instrument | Interest rate swap, Expired, 2018 secured railcar equipment notes | AOCL – loss/ (income)
Derivative [Line Items]
Total stockholders' equity	1.4
Designated as hedging instrument | Interest rate swap, Expired, 2018 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	3.5
Designated as hedging instrument | Interest rate swap, Expired, TRIP Holdings warehouse loan | Noncontrolling Interest
Derivative [Line Items]
Total stockholders' equity	4.7
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	0.4
Designated as hedging instrument | Interest rate cap, Open, 2017 promissory note
Derivative [Line Items]
Notional Amount	$ 171.6
Libor plus margin for 2017 promissory note	3.00%
Derivative asset	$ 2.7
Designated as hedging instrument | Interest rate cap, Open, 2017 promissory note | AOCL – loss/ (income)
Derivative [Line Items]
Total stockholders' equity	(0.4)
Designated as hedging instrument | Interest rate cap, Open, 2017 promissory note | Noncontrolling Interest
Derivative [Line Items]
Total stockholders' equity	$ 0.0

[1] (unaudited)