Commitments, Guarantees, and Contingencies (Narrative) (Details) (USD $)	3 Months Ended				0 Months Ended			1 Months Ended	3 Months Ended	12 Months Ended					108 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Aug. 31, 2009 customer	Mar. 31, 2013 Legal Reserve	Aug. 31, 2009 Securities Finance	Mar. 31, 2013 Securities Finance	Sep. 30, 2010 Securities Finance	Oct. 31, 2010 Foreign Exchange	Mar. 31, 2013 Foreign Exchange	Dec. 31, 2012 Foreign Exchange	Dec. 31, 2011 Foreign Exchange	Dec. 31, 2010 Foreign Exchange	Dec. 31, 2009 Foreign Exchange	Dec. 31, 2008 Foreign Exchange	Dec. 31, 2009 Foreign Exchange	Mar. 31, 2013 Shareholder Litigation complaint shareholder
Loss Contingencies [Line Items]
Unfunded commitments to extend credit	$ 18,280,000,000	$ 17,860,000,000
Unfunded commitments to extend credit, short term	77.00%
Percent of fair market value that must be maintained in collateral	100.00%
Cash collateral provided for securities lending	9,090,000,000	6,830,000,000
Cash collateral received for securities lending	7,110,000,000	4,990,000,000
Number of suits from related participants of agency securities lending program			2
Loss contingency, damages sought, value					120,000,000
The difference between the amortized cost and market value of the in-kind distribution						10,000,000	49,000,000
Litigation reserve						10,000,000
Actual damages asserted by attorney general															100,000,000
Settlement, amount								12,000,000
Estimated indirect foreign exchange revenue									65,000,000	248,000,000	331,000,000	336,000,000	369,000,000	462,000,000
Shareholder related class action complaints pending in Boston Federal Court																4
Number of class action complaints pending in Boston Federal Court brought on behalf of State Street shareholders																1
Number of class action complaints pending in Boston Federal Court brought on behalf of State Street participants and benficiaries of State Street Salary Savings Program																2
Legal reserves				10,000,000
Unrecognized tax benefits	$ 95,000,000	$ 95,000,000