Business Segment (Segment Profit and Assets) (Details) - USD ($) $ in Millions	3 Months Ended										12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016		Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Segment Reporting Information [Line Items]
Interest income	$ 447.7		$ 454.0	$ 478.2	$ 455.7	$ 474.1		$ 475.7	$ 478.7	$ 482.9	$ 1,835.6		$ 1,911.5		$ 1,445.2
Interest expense (benefit)	168.7		176.7	209.2	163.1	178.3		188.2	191.6	195.0	717.7		753.2		731.4
Provision for credit losses	30.4		30.1	4.4	49.7	36.7		45.1	23.3	89.5	114.6		194.7		158.6
Rental income on operating leases	252.6		252.3	251.2	251.3	252.2		254.3	261.0	264.1	1,007.4		1,031.6		1,018.1
Other non-interest income											364.2		150.6		149.6
Depreciation on operating lease equipment	74.3		71.1	77.4	73.5	69.8		66.9	63.1	61.3	296.3		261.1		229.2
Maintenance and other operating lease expenses	57.9		57.9	53.3	53.8	57.5		56.6	50.6	48.9	222.9		213.6		185.1
Goodwill impairment	255.6		$ 0.0	$ 0.0	$ 0.0	354.2		$ 0.0	$ 0.0	$ 0.0	255.6		354.2		0.0
Operating expenses / loss on debt extinguishment											1,408.5		1,296.0		1,122.6
Income from continuing operations before (benefit) provision for income taxes											191.6		20.9		186.0
Total loans	29,113.9					29,535.9					29,113.9		29,535.9		30,518.7
Credit balances of factoring clients	(1,468.6)					(1,292.0)					(1,468.6)		(1,292.0)		(1,344.0)
Assets held for sale	2,263.1	[1]				636.0	[1]				2,263.1	[1]	636.0	[1]	2,057.7
Operating lease equipment, net	6,738.9	[1]				7,486.1	[1]				6,738.9	[1]	7,486.1	[1]	6,851.7
Commercial Banking
Segment Reporting Information [Line Items]
Total loans	23,159.3					22,562.3					23,159.3		22,562.3
Consumer Banking
Segment Reporting Information [Line Items]
Total loans	5,954.6					6,973.6					5,954.6		6,973.6
Non-Strategic Portfolios
Segment Reporting Information [Line Items]
Goodwill impairment											255.6		354.2
Operating Segments | Commercial Banking
Segment Reporting Information [Line Items]
Interest income											1,248.0		1,287.9		1,029.1
Interest expense (benefit)											517.7		519.1		481.4
Provision for credit losses											88.7		183.1		143.7
Rental income on operating leases											1,007.4		1,020.0		981.4
Other non-interest income											291.0		293.8		302.6
Depreciation on operating lease equipment											296.3		261.1		218.3
Maintenance and other operating lease expenses											222.9		213.6		185.1
Goodwill impairment											255.6		34.8
Operating expenses / loss on debt extinguishment											691.7		761.6		727.4
Income from continuing operations before (benefit) provision for income taxes											473.5		628.4		557.2
Total loans	23,159.3					22,562.3					23,159.3		22,562.3		23,332.4
Credit balances of factoring clients	(1,468.6)					(1,292.0)					(1,468.6)		(1,292.0)		(1,344.0)
Assets held for sale	1,334.2					357.7					1,334.2		357.7		435.1
Operating lease equipment, net	6,738.9					7,486.1					6,738.9		7,486.1		6,851.7
Operating Segments | Consumer Banking
Segment Reporting Information [Line Items]
Interest income											378.1		420.8		176.1
Interest expense (benefit)											(51.8)		10.2		24.9
Provision for credit losses											25.9		11.7		8.7
Rental income on operating leases											0.0		0.0		0.0
Other non-interest income											4.1		40.0		5.4
Depreciation on operating lease equipment											0.0		0.0		0.0
Maintenance and other operating lease expenses											0.0		0.0		0.0
Goodwill impairment											0.0		319.4
Operating expenses / loss on debt extinguishment											401.5		380.9		158.4
Income from continuing operations before (benefit) provision for income taxes											6.6		(261.4)		(10.5)
Total loans	5,954.6					6,973.6					5,954.6		6,973.6		7,186.3
Credit balances of factoring clients	0.0					0.0					0.0		0.0		0.0
Assets held for sale	865.6					68.2					865.6		68.2		45.1
Operating lease equipment, net	0.0					0.0					0.0		0.0		0.0
Operating Segments | Non-Strategic Portfolios
Segment Reporting Information [Line Items]
Interest income											22.9		80.8		184.8
Interest expense (benefit)											15.2		47.2		121.4
Provision for credit losses											0.0		(0.1)		6.2
Rental income on operating leases											0.0		11.6		36.7
Other non-interest income											3.1		52.1		(96.8)
Depreciation on operating lease equipment											0.0		0.0		10.9
Maintenance and other operating lease expenses											0.0		0.0		0.0
Goodwill impairment											0.0		0.0
Operating expenses / loss on debt extinguishment											12.7		42.2		123.9
Income from continuing operations before (benefit) provision for income taxes											(1.9)		55.2		(137.7)
Total loans	0.0					0.0					0.0		0.0		0.0
Credit balances of factoring clients	0.0					0.0					0.0		0.0		0.0
Assets held for sale	63.3					210.1					63.3		210.1		1,577.5
Operating lease equipment, net	0.0					0.0					0.0		0.0		0.0
Corporate & Other
Segment Reporting Information [Line Items]
Interest income											186.6		122.0		55.2
Interest expense (benefit)											236.6		176.7		103.7
Provision for credit losses											0.0		0.0		0.0
Rental income on operating leases											0.0		0.0		0.0
Other non-interest income											66.0		(235.3)		(61.6)
Depreciation on operating lease equipment											0.0		0.0		0.0
Maintenance and other operating lease expenses											0.0		0.0		0.0
Goodwill impairment											0.0		0.0
Operating expenses / loss on debt extinguishment											302.6		111.3		112.9
Income from continuing operations before (benefit) provision for income taxes											(286.6)		(401.3)		(223.0)
Total loans	0.0					0.0					0.0		0.0		0.0
Credit balances of factoring clients	0.0					0.0					0.0		0.0		0.0
Assets held for sale	0.0					0.0					0.0		0.0		0.0
Operating lease equipment, net	$ 0.0					$ 0.0					$ 0.0		$ 0.0		$ 0.0

[1] The following table presents information on assets and liabilities related to Variable Interest Entities (VIEs) that are consolidated by the Company. The difference between VIE total assets and total liabilities represents the Company's interests in those entities, which were eliminated in consolidation. The assets of the consolidated VIEs will be used to settle the liabilities of those entities and, except for the Company's interest in the VIEs, are not available to the creditors of CIT or any affiliates of CIT.