Fair Value (Narrative) (Details) $ in Millions		3 Months Ended										12 Months Ended
	Aug. 03, 2015 USD ($) holdback	Dec. 31, 2017 USD ($)		Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)		Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)		Dec. 31, 2016 USD ($)		Dec. 31, 2015 USD ($)
Fair Value Disclosure [Line Items]
Borrower rate, max effective period												90 days
Goodwill impairment		$ 255.6		$ 0.0	$ 0.0	$ 0.0	$ 354.2		$ 0.0	$ 0.0	$ 0.0	$ 255.6		$ 354.2		$ 0.0
Assets held for sale		2,263.1	[1]				636.0	[1]				$ 2,263.1	[1]	$ 636.0	[1]	$ 2,057.7
Fair value of loans as a percentage of carrying value												101.60%		100.60%
Unpaid Principal Balance		3,065.7					3,685.0					$ 3,065.7		$ 3,685.0
Impaired loans with related allowance carrying amount		146.7					188.2					$ 146.7		$ 188.2
Impaired loans with related allowance carrying amount as a percentage of unpaid principal balance												75.00%		77.00%
Impaired loans with no specific allowance unpaid principal balance		86.1					74.7					$ 86.1		$ 74.7
Impaired loans with no specific allowance carrying value		63.6					55.5					63.6		55.5
Estimated Fair Value
Fair Value Disclosure [Line Items]
Loans (excluding leases)		26,844.8					26,846.7					26,844.8		26,846.7
Impaired Loans
Fair Value Disclosure [Line Items]
Unpaid Principal Balance		195.5					244.3					195.5		244.3
Level 2
Fair Value Disclosure [Line Items]
Assets held for sale		4.7										4.7
Unsecured debt		3,800.0					10,600.0					3,800.0		10,600.0
Par value of debt		4,300.0					3,300.0					4,300.0		3,300.0
Level 2 | Estimated Fair Value
Fair Value Disclosure [Line Items]
Loans (excluding leases)		624.3					390.3					624.3		390.3
Level 3
Fair Value Disclosure [Line Items]
Par value of debt		1,000.0					1,100.0					1,000.0		1,100.0
Level 3 | Estimated Fair Value
Fair Value Disclosure [Line Items]
Loans (excluding leases)		26,220.5					26,456.4					26,220.5		$ 26,456.4
OneWest Bank
Fair Value Disclosure [Line Items]
Probable amount of holdback to be paid												62.4
Consideration holdback liability		46.0										46.0
Goodwill impairment							319.4
OneWest Bank | Minimum
Fair Value Disclosure [Line Items]
Range of potential holdback to be paid		0.0										0.0
OneWest Bank | Maximum
Fair Value Disclosure [Line Items]
Range of potential holdback to be paid		116.0										$ 116.0
FDIC True-up liability | Level 3
Fair Value Disclosure [Line Items]
Discount Rate												2.90%		3.20%
FDIC True-up liability | Weighted Average | Level 3
Fair Value Disclosure [Line Items]
Discount Rate												2.90%		3.20%
Consideration holdback liability | Minimum | Level 3
Fair Value Disclosure [Line Items]
Discount Rate														1.30%
Consideration holdback liability | Maximum | Level 3
Fair Value Disclosure [Line Items]
Discount Rate														4.00%
Consideration holdback liability | Weighted Average | Level 3
Fair Value Disclosure [Line Items]
Discount Rate														2.10%
Consideration holdback liability | OneWest Bank
Fair Value Disclosure [Line Items]
Number of seperate holdbacks | holdback	4
Reduction in cash consideration due to trailing risks	$ 116.0
Consideration holdback liability | OneWest Bank | Minimum
Fair Value Disclosure [Line Items]
Holdback periods												1 year
Consideration holdback liability | OneWest Bank | Maximum
Fair Value Disclosure [Line Items]
Holdback periods												5 years
Other Real Estate Owned | Minimum | Level 3
Fair Value Disclosure [Line Items]
Costs to sell												5.40%
Other Real Estate Owned | Maximum | Level 3
Fair Value Disclosure [Line Items]
Costs to sell												52.60%
Other Real Estate Owned | Weighted Average | Level 3
Fair Value Disclosure [Line Items]
Costs to sell												6.50%
Securities carried at fair value with changes recorded in net income | Level 3
Fair Value Disclosure [Line Items]
Discount Rate												31.10%
Securities carried at fair value with changes recorded in net income | Minimum | Level 3
Fair Value Disclosure [Line Items]
Discount Rate														0.00%
Securities carried at fair value with changes recorded in net income | Maximum | Level 3
Fair Value Disclosure [Line Items]
Discount Rate														34.60%
Securities carried at fair value with changes recorded in net income | Weighted Average | Level 3
Fair Value Disclosure [Line Items]
Discount Rate												31.10%		5.60%
Consumer Banking Reporting Unit
Fair Value Disclosure [Line Items]
Goodwill impairment							$ 319.4
Commercial Services
Fair Value Disclosure [Line Items]
Goodwill impairment		$ 8.6										$ 8.6		$ 34.8

[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.