Fair Value Measurement and Election - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Estimated Fair Value of Loan Portfolio's Net Carrying Value, Percentage	100.00%
Assets	$ 189,881,000,000us-gaap_Assets		$ 190,328,000,000us-gaap_Assets
Loans Receivable, Fair Value Disclosure	268,000,000		272,000,000
Allowance for Loan and Lease Losses, Write-offs	130,000,000	151,000,000
Loans held for investment sold	405,000,000sti_LoansHeldForInvestmentSold	85,000,000sti_LoansHeldForInvestmentSold
Gain (Loss) on Sales of Loans, Net	6,000,000us-gaap_GainLossOnSalesOfLoansNet	9,000,000us-gaap_GainLossOnSalesOfLoansNet
Unfunded loan commitments and letters of credit	57,600,000,000		56,500,000,000
Allowance for unfunded loan commitments and letters of credit	61,000,000		59,000,000
Total Return Swap \| Fair Value, Inputs, Level 2 [Member]
Loans Receivable, Fair Value Disclosure	2,300,000,000
Interest Rate Lock Commitments [Member]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	60,000,000	55,000,000
Trading Account Assets [Member] \| Commercial and Corporate Leveraged Loans [Member] \| Fair Value, Inputs, Level 2 [Member]
Loans Receivable, Fair Value Disclosure	394,000,000		284,000,000
Long-term Debt [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Option, Credit Risk, Gains (Losses) on Liabilities	(4,000,000)	17,000,000
Commercial and Corporate Loans [Member] \| Variable Interest Entity, Not Primary Beneficiary [Member]
Assets	677,000,000		704,000,000
Commercial and Corporate Loans [Member] \| Variable Interest Entity, Not Primary Beneficiary [Member] \| Preference Shares [Member]
Assets	3,000,000		3,000,000
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Loans Receivable, Fair Value Disclosure	0		0
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member]
Loans Receivable, Fair Value Disclosure	268,000,000		272,000,000
Other Real Estate Owned [Member] \| Fair Value, Measurements, Nonrecurring [Member]
Asset Impairment Charges	3,000,000		6,000,000
Other Assets [Member] \| Fair Value, Measurements, Nonrecurring [Member]
Asset Impairment Charges	0		64,000,000
Other Assets [Member] \| Fair Value, Measurements, Nonrecurring [Member] \| Land [Member]
Asset Impairment Charges	0		5,000,000
Other Assets [Member] \| Fair Value, Measurements, Nonrecurring [Member] \| Property Subject to Operating Lease [Member]
Asset Impairment Charges			59,000,000
Affordable Housing [Member]
Asset Impairment Charges			(15,000,000)
Affordable Housing [Member] \| Fair Value, Measurements, Nonrecurring [Member]
Asset Impairment Charges		$ 36,000,000	$ 21,000,000

Allowance for unfunded loan commitments and letters of credit

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- Definition

Percentage of Carrying Value

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- Definition

Carring Amount of Loans Held For Investment Sold

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- Definition

Unfunded loan commitments and letters of credit

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- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

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-Name Accounting Standards Codification

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Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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- Definition

Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

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- Definition

This item represents the estimated amount of gains and losses from fair value changes included in earnings that are attributable to changes in the instrument-specific credit risk of liabilities with fair values that have been significantly affected during the reporting period by changes in the instrument-specific credit risk.

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-Topic 825

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-Paragraph 30

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The net gain (loss) resulting from a sale of loans, including adjustments to record loans classified as held-for-sale at the lower-of-cost-or-market and fair value adjustments to loan held for investment purposes.

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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

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-SubTopic 10

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