Borrowings (Narrative) (Details)		3 Months Ended
		Mar. 31, 2020 USD ($) subsidiary	Mar. 31, 2019	Dec. 31, 2019 USD ($)	Apr. 04, 2017
Debt Instrument [Line Items]
Tier 1 Capital minimum ratio	[1]	8.50%		8.50%
Pledged assets		$ 15,800,000,000
Pledged assets, loans		12,800,000,000
Pledged assets, investments		3,000,000,000.0
FHLB advances, financing availability		8,758,500,000		$ 6,350,500,000
FHLB advances, unused and available		$ 5,708,500,000		$ 4,700,500,000
FHLB advances, weighted average percentage rates		1.03%		2.04%
FHLB advances		$ 3,050,000,000		$ 1,650,000,000
Pledged assets		10,314,300,000		6,987,600,000
Pledged assets		2,388,500,000		2,205,900,000
Long-term borrowings		8,094,300,000		6,473,400,000
Federal Reserve System ("FRB")
Debt Instrument [Line Items]
Long-term borrowings		0		0
CIT-owned Subsidiaries
Debt Instrument [Line Items]
Secured borrowings		561,300,000		$ 361,100,000
Long-term borrowings		$ 4,171,400,000
Other secured and structured financings
Debt Instrument [Line Items]
Secured borrowings, weighted average percentage rate		2.27%		3.03%
Long-term borrowings		$ 561,300,000		$ 361,100,000
Other secured and structured financings | CIT-owned Subsidiaries
Debt Instrument [Line Items]
Long-term borrowings		561,300,000
High quality liquid securities
Debt Instrument [Line Items]
FHLB advances, financing availability		2,879,500,000
FHLB advances, unused and available		2,879,500,000
FHLB amount of advance		3,006,300,000
Pledged assets		$ 3,006,300,000		$ 50,400,000
Maximum | Other secured and structured financings
Debt Instrument [Line Items]
Secured borrowings, weighted average percentage rate		3.58%
Minimum | Other secured and structured financings
Debt Instrument [Line Items]
Secured borrowings, weighted average percentage rate		2.24%
Weighted Avg.
Debt Instrument [Line Items]
FHLB advances, weighted average percentage rates		1.03%		2.04%
Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility, total commitment amount		$ 300,000,000		$ 400,000,000
Tier 1 Capital minimum ratio					8.50%
Revolving credit facility, domestic operating subsidiary guarantors | subsidiary		3
Revolving credit facility, maturity date			Mar. 01, 2021
Revolving credit facility, extended maturity date			Nov. 01, 2021
Revolving credit facility, minimum guarantor asset coverage ratio		1.25
Revolving Credit Facility | Maximum
Debt Instrument [Line Items]
Revolving credit facility, minimum guarantor asset coverage ratio		1.5
Revolving Credit Facility | Minimum
Debt Instrument [Line Items]
Revolving credit facility, minimum guarantor asset coverage ratio		1.0
Revolving Credit Facility | LIBOR | Maximum
Debt Instrument [Line Items]
Debt, applicable margin (percentage)		1.75%
Revolving Credit Facility | Base Rate | Maximum
Debt Instrument [Line Items]
Debt, applicable margin (percentage)		0.75%
Letters of Credit
Debt Instrument [Line Items]
Revolving credit facility, total commitment amount		$ 100,000,000
Outstanding borrowings		40,000,000
Revolving Credit Facility, Not Letter of Credit
Debt Instrument [Line Items]
Revolving credit facility, total commitment amount		$ 200,000,000

[1] ^{Required minimum ratios include stated minimums of 4.5%, 6% and 8% for CET1 capital, Tier 1 capital and Total capital ratios, respectively, plus the fully phased-in capital conservation buffer of 2.5%}