Fair Value Fair Value (Narrative) (Details)	Mar. 31, 2012
Fair Value Disclosures [Abstract]
Percent of Level 3 Portfolio Priced Using Non-Binding Broker Quotes	92.00%
Percent of Level 3 Portfolio Priced Using Internal Model Pricing	8.00%

Percent of Level 3 Portfolio Priced Using Non-Binding Broker Quotes

No definition available.

Name:	stt_PercentOfLevel3PortfolioPricedUsingNonBindingBrokerQuotes
Namespace Prefix:	stt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant