Borrowings (Narrative) (Details)				3 Months Ended		9 Months Ended
	Sep. 28, 2017 USD ($)	May 04, 2017 USD ($)	Apr. 18, 2017 USD ($)	Sep. 30, 2017 USD ($) subsidiary	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) subsidiary	Sep. 30, 2016 USD ($)	Sep. 15, 2017 USD ($)	Apr. 04, 2017 USD ($)	Apr. 03, 2017 USD ($) subsidiary	Feb. 28, 2017 USD ($)	Jan. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Tier 1 Capital minimum ratio				7.25%		7.25%							6.625%
Revolving credit facility, domestic operating subsidiary guarantors | subsidiary				8		8				9
Revolving credit facility, minimum guarantor asset coverage ratio				1.25		1.25
Revolving credit facility, available draw amount				$ 675,000,000		$ 675,000,000
Debt repurchased		$ 4,840,000,000
Loss on debt extinguishment and deposit redemption				(53,500,000)	$ (5,200,000)	$ (218,300,000)	$ (9,200,000)
Additional premium to purchase price		$ 98,000,000
Senior unsecured notes, percent of purchase price to principal amount						101.00%
Pledged assets				13,900,000,000		$ 13,900,000,000
Pledged assets, loans				12,400,000,000		12,400,000,000
Pledged assets, operating lease assets				1,200,000,000		1,200,000,000
Pledged assets, cash				200,000,000		200,000,000
Pledged assets, investments				100,000,000		100,000,000
FHLB advances, financing availability				4,900,000,000		4,900,000,000
FHLB advances, unused and available				1,700,000,000		1,700,000,000
Secured borrowings				1,637,700,000		1,637,700,000							$ 1,925,700,000
Onewest Bank | Variable Interest Entities | HMBS Sensitizations
Debt Instrument [Line Items]
Secured borrowings				143,800,000		143,800,000							160,200,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Revolving credit facility, total commitment amount				750,000,000		750,000,000					$ 1,400,000,000.0	$ 1,500,000,000.0
Revolving Credit Facility [Member] | CIT Commercial Air Business
Debt Instrument [Line Items]
Revolving credit facility, total commitment amount									$ 750,000,000
Revolving Credit Facility [Member] | CIT Commercial Air Business | Discontinued Operations, Disposed of by Sale
Debt Instrument [Line Items]
Revolving credit facility, total commitment amount										$ 1,400,000,000.0
Minimum consolidated net worth covenant										$ 6,000,000,000
Tier 1 Capital minimum ratio									9.00%
Letters of Credit
Debt Instrument [Line Items]
Revolving credit facility, available draw amount				75,000,000		75,000,000
FHLB advances, utilized for issuance of letters of credit				65,900,000		65,900,000
Discontinued Operations | Onewest Bank
Debt Instrument [Line Items]
Secured borrowings				$ 293,600,000		$ 293,600,000							$ 1,571,000,000
Maximum
Debt Instrument [Line Items]
Revolving credit facility, minimum guarantor asset coverage ratio				1.0		1.0
Minimum
Debt Instrument [Line Items]
Revolving credit facility, minimum guarantor asset coverage ratio				1.5		1.5
Senior unsecured notes, percent of purchase price to principal amount						98.00%
Weighted Average
Debt Instrument [Line Items]
FHLB advances, weighted average percentage rate				1.44%		1.44%
August 2017 - 4.250%
Debt Instrument [Line Items]
Percent of principal amounts redeemed		100.00%
Tender Offer
Debt Instrument [Line Items]
Principal amount repurchased	$ 800,000,000
Debt repurchased	861,200,000
Unamortized premium balance	50,600,000		$ 59,000,000
Accrued interest	9,300,000		9,000,000
Tender Offer | Maximum
Debt Instrument [Line Items]
Principal amount repurchased								$ 800,000,000	$ 950,000,000
February 2019 - 5.500%
Debt Instrument [Line Items]
Principal amount repurchased			969,000,000
Debt repurchased	398,000,000		$ 1,040,000,000
May 2020 - 5.375%
Debt Instrument [Line Items]
Debt repurchased	302,000,000
August 2022 - 5.000%
Debt Instrument [Line Items]
Debt repurchased	$ 100,000,000
Senior Unsecured Note, 6.00% Maturing in 2036
Debt Instrument [Line Items]
Face amount				$ 51,000,000		$ 51,000,000
Rate (%)				6.00%		6.00%
Carrying value				$ 39,500,000		$ 39,500,000
Secured Borrowings
Debt Instrument [Line Items]
Secured borrowings, weighted average percentage rate				3.62%		3.62%
Secured Borrowings | Maximum
Debt Instrument [Line Items]
Secured borrowings, weighted average percentage rate				5.74%		5.74%
Secured Borrowings | Minimum
Debt Instrument [Line Items]
Secured borrowings, weighted average percentage rate				0.55%		0.55%
Senior Unsecured Notes
Debt Instrument [Line Items]
Face amount				$ 3,731,000,000		$ 3,731,000,000
Senior Unsecured Notes | August 2017 - 4.250%
Debt Instrument [Line Items]
Debt repurchased		$ 1,725,800,000
Rate (%)		4.25%
Senior Unsecured Notes | March 2018 - 5.250%
Debt Instrument [Line Items]
Debt repurchased		$ 1,465,000,000
Rate (%)		5.25%
Senior Unsecured Notes | April 2018 - 6.625%
Debt Instrument [Line Items]
Debt repurchased		$ 695,000,000
Rate (%)		6.625%
Senior Unsecured Notes | May 2018 - 5.000%
Debt Instrument [Line Items]
Debt repurchased		$ 955,900,000
Rate (%)		5.00%
Senior Unsecured Notes | February 2019 - 5.500%
Debt Instrument [Line Items]
Face amount				$ 383,000,000		$ 383,000,000
Rate (%)	5.50%			5.50%		5.50%		5.50%	5.50%
Senior Unsecured Notes | May 2020 - 5.375%
Debt Instrument [Line Items]
Face amount				$ 448,000,000		$ 448,000,000
Rate (%)	5.375%			5.375%		5.375%		5.375%	5.375%
Senior Unsecured Notes | August 2022 - 5.000%
Debt Instrument [Line Items]
Face amount				$ 1,150,000,000		$ 1,150,000,000
Rate (%)	5.00%			5.00%		5.00%		5.00%	5.00%
LIBOR | Maximum | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Revolving credit facility, applicable margin (percentage)						2.00%
Base Rate | Maximum | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Revolving credit facility, applicable margin (percentage)						1.00%