Fair Value Measurement and Election - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Unfunded loan commitments and letters of credit	$ 76,200	$ 72,000
Allowance for unfunded loan commitments and letters of credit	$ 76	$ 72

Allowance for commercial unfunded loan commitments and letters of credit

No definition available.

Name:	sti_AllowanceforCommercialUnfundedLoanCommitmentsandLettersofCredit
Namespace Prefix:	sti_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant