Investment Securities (Narrative) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2017
New Accounting Pronouncement, Early Adoption [Line Items]
Available-for-sale securities		$ 6,168,300,000	$ 3,708,200,000
Realized investment gains excluding losses from OTTI		33,900,000	29,300,000	$ 8,100,000
Interest bearing deposits	[1]	1,440,100,000	5,608,500,000
Estimated fair value of PCI securities		312,500,000	478,900,000
Par value of PCI securities		387,600,000	615,200,000
Securities carried at fair value with changes recorded in net income, fair value		400,000	283,500,000
Securities carried at fair value with changes recorded in net income, amortized cost			277,500,000
Securities carried at fair value with changes recorded in net income, unrealized holding loss		0	700,000
Securities carried at fair value with changes recorded in net income, unrealized holding gains			6,700,000
Securities carried at fair value with changes recorded in net income, weighted average yield					41.80%
OTTI credit-related losses, PCI securities		1,100,000	3,300,000	$ 2,800,000
OTTI unrealized losses on non-marketable investments		0
Debt securities
New Accounting Pronouncement, Early Adoption [Line Items]
Held-to-maturity securities		0	243,000,000
Available-for-sale securities		6,123,600,000	3,674,100,000
Securities carried at fair value with changes recorded in net income, fair value		$ 400,000	283,500,000
Debt securities | Accounting Standards Update 2017-12 | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncement, Early Adoption [Line Items]
Held-to-maturity securities			(207,000,000)
Available-for-sale securities			$ 207,000,000

[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.