Discontinued Operations (Narrative) (Details) - USD ($) $ in Millions				12 Months Ended
		May 16, 2017	Apr. 04, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of discontinued operations, net of taxes				$ 118.6	$ 0.0	$ 0.0
Loan portfolio	[1]			28,682.8	29,103.3
Indemnification assets				142.4	341.4
Settled Litigation | HUD OIG Investigation | Unfavorable Regulatory Action
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Settlement		$ 89.0
Consumer Banking | Operating Segments | Reverse Mortgage | OneWest Bank
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loan portfolio				861.0
Real estate owned assets				21.0
Discontinued Operations, Disposed of by Sale | Commercial Air
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase price of business sold			$ 10,400.0
Gain on business sold			146.0
Gain on sale of discontinued operations, net of taxes			$ 106.0
Discontinued Operations, Disposed of by Sale | Aerospace
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of discontinued operations, net of taxes				118.6	0.0	0.0
Incremental pretax amount of indirect overhead expense in continuing operations					19.0	39.0
Discontinued Operation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of discontinued operations, net of taxes				118.6	0.0	$ 0.0
Discontinued Operation | Financial Freedom | Settled Litigation | HUD OIG Investigation | Unfavorable Regulatory Action
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Settlement				89.0
Discontinued Operation | Financial Freedom | Residential Mortgage
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Indemnification receivable released				77.0
Indemnification assets				29.0
Net release of curtailment reserve				(111.0)
Increase in other servicing related reserves				40.0
Impairment charge to servicing liability				$ 50.0	$ 19.0

[1] The following table presents information on assets and liabilities related to Variable Interest Entities (VIEs) that are consolidated by the Company. The difference between VIE total assets and total liabilities represents the Company's interests in those entities, which were eliminated in consolidation. The assets of the consolidated VIEs will be used to settle the liabilities of those entities and, except for the Company's interest in the VIEs, are not available to the creditors of CIT or any affiliates of CIT.