Regulatory Capital (Tier 1 Capital And Total Capital Components) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 Capital	$ 6,479.8	$ 9,058.9
Tier 1 Capital	6,775.4	9,058.9
Total Capital	7,251.0	9,535.2
Risk-Weighted Assets	$ 44,537.7	$ 64,586.3
Common Equity Tier 1 Capital Ratio
Actual	14.50%	14.00%
Effective minimum ratios under Basel III guidelines	5.75%	5.125%
Tier 1 Capital Ratio:
Actual	15.20%	14.00%
Effective minimum ratios under Basel III guidelines	7.25%	6.625%
Total Capital Ratio:
Actual	16.30%	14.80%
Effective minimum ratios under Basel III guidelines	9.25%	8.625%
Tier 1 Leverage Ratio:
Actual	13.80%	13.90%
Required minimum ratio for capital adequacy purposes	4.00%	4.00%
CIT Bank, N.A.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 Capital	$ 4,751.6	$ 4,623.2
Tier 1 Capital	4,751.6	4,623.2
Total Capital	5,183.3	5,053.4
Risk-Weighted Assets	$ 34,527.2	$ 34,410.3
Common Equity Tier 1 Capital Ratio
Actual	13.80%	13.40%
Effective minimum ratios under Basel III guidelines	5.75%	5.125%
Tier 1 Capital Ratio:
Actual	13.80%	13.40%
Effective minimum ratios under Basel III guidelines	7.25%	6.625%
Total Capital Ratio:
Actual	15.00%	14.70%
Effective minimum ratios under Basel III guidelines	9.25%	8.625%
Tier 1 Leverage Ratio:
Actual	11.80%	10.90%
Required minimum ratio for capital adequacy purposes	4.00%	4.00%