Parent Company Financial Statements (Condensed Parent Company Only Statement Of Operations And Comprehensive Income) (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
Income
Interest income from nonbank subsidiaries	$ 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 and dividends on interest bearing deposits and investments									197.5	131.9	71.2
Other non-interest income	137.2	63.3	84.6	79.1	(117.6)	83.6	99.8	84.8	364.2	150.6	149.6
Expenses
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
Total non-interest expenses									2,183.3	2,124.9	1,536.9
Income from continuing operations before (benefit) provision for income taxes									191.6	20.9	186.0
Continuing operations (benefit) / provision	27.7	(119.8)	(31.9)	56.2	(6.6)	54.5	111.2	44.4	(67.8)	203.5	(538.0)
Net income (loss)									468.2	(848.0)	1,034.1
Net income (loss)	$ (88.0)	$ 219.6	$ 156.7	$ 179.9	$ (1,142.5)	$ 131.5	$ 17.0	$ 146.0	468.2	(848.0)	1,034.0
Other Comprehensive income (loss), net of tax									53.6	2.0	(8.2)
Comprehensive income (loss)									521.8	(846.0)	1,025.9
CIT Group Inc.
Income
Interest income from nonbank subsidiaries									160.5	488.3	435.1
Interest and dividends on interest bearing deposits and investments									7.4	2.7	3.2
Dividends from nonbank subsidiaries									0.0	399.9	630.3
Dividends from bank subsidiaries									359.0	223.0	459.2
Other non-interest income from subsidiaries									194.0	146.3	(138.8)
Other non-interest income									(127.9)	21.0	128.8
Total income									593.0	1,281.2	1,517.8
Expenses
Interest expense (benefit)									324.7	548.2	570.7
Interest expense on liabilities to subsidiaries									(50.3)	(51.1)	(43.9)
Other non-interest expenses									(499.4)	(565.0)	(267.2)
Total non-interest expenses									874.4	1,164.3	881.8
Income from continuing operations before (benefit) provision for income taxes									(281.4)	116.9	636.0
Continuing operations (benefit) / provision									163.4	(308.5)	(827.2)
Net income (loss)									(444.8)	425.4	1,463.2
Equity in undistributed net income of bank subsidiaries									(55.6)	(349.8)	(265.1)
Equity in undistributed net income of nonbank subsidiaries									968.6	(923.6)	(164.0)
Net income (loss)									468.2	(848.0)	1,034.1
Other Comprehensive income (loss), net of tax									53.6	2.0	(8.2)
Comprehensive income (loss)									$ 521.8	$ (846.0)	$ 1,025.9