Parent Company Financial Statements (Condensed Parent Company Only Statements Of Cash Flows) (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
Cash Flows From Operations
Net income (loss) before attribution of noncontrolling interests	$ (88.0)	$ 219.6	$ 156.7	$ 179.9	$ (1,142.5)	$ 131.5	$ 17.0	$ 146.0	$ 468.2	$ (848.0)	$ 1,034.0
Net cash flows provided by operations									519.8	2,399.2	1,189.8
Cash Flows From Investing Activities
Acquisitions									0.0	0.0	2,521.2
Net cash flows provided by (used in) investing activities									7,755.2	(495.3)	1,085.2
Cash Flows From Financing Activities
Proceeds from the issuance of term debt									15.3	781.1	1,626.9
Repayments of term debt and net settlements									(8,427.3)	(2,619.0)	(4,325.3)
Net proceeds from issuance of preferred stock									318.0	0.0	0.0
Repurchase of common stock									(3,431.9)	0.0	(531.8)
Dividends paid									(113.7)	(123.0)	(114.9)
Net cash flows used in financing activities									(13,071.8)	(2,964.7)	(896.1)
(Decrease) increase in unrestricted cash and cash equivalents									(4,781.2)	(1,095.4)	1,315.1
Unrestricted cash and cash equivalents, beginning of period				6,375.2				7,470.6	6,375.2	7,470.6	6,155.5
Unrestricted cash and cash equivalents, end of period	1,594.0				6,375.2				1,594.0	6,375.2	7,470.6
CIT Group Inc.
Cash Flows From Operations
Net income (loss) before attribution of noncontrolling interests									468.2	(848.0)	1,034.1
Equity in undistributed earnings of subsidiaries									(1,272.0)	650.4	429.1
Other operating activities, net									621.5	69.0	(566.4)
Net cash flows provided by operations									(182.3)	(128.6)	896.8
Cash Flows From Investing Activities
Decrease in investments in subsidiaries									2,096.7	1,023.1	620.1
Acquisitions									0.0	0.0	(1,559.5)
Decrease (increase) in Investment securities and securities purchased under agreements to resell									250.3	(100.2)	1,454.1
Net cash flows provided by (used in) investing activities									2,347.0	922.9	514.7
Cash Flows From Financing Activities
Repayments of term debt and net settlements									(7,087.7)	(359.5)	(1,256.7)
Net proceeds from issuance of preferred stock									318.0	0.0	0.0
Repurchase of common stock									(3,431.9)	0.0	(531.8)
Dividends paid									(113.7)	(123.0)	(114.9)
Net change in advances from (to) subsidiaries									7,759.9	(131.5)	91.0
Other financing activities, net									(20.7)	(21.9)	(22.2)
Net cash flows used in financing activities									(2,576.1)	(635.9)	(1,834.6)
(Decrease) increase in unrestricted cash and cash equivalents									(411.4)	158.4	(423.1)
Unrestricted cash and cash equivalents, beginning of period				$ 1,188.2				$ 1,029.8	1,188.2	1,029.8	1,452.9
Unrestricted cash and cash equivalents, end of period	$ 776.8				$ 1,188.2				$ 776.8	$ 1,188.2	$ 1,029.8