Shareholders' Equity (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	6 Months Ended		9 Months Ended		12 Months Ended
	Mar. 31, 2011	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Dec. 31, 2009
Non-cumulative perpetual preferred stock, shares	5,001	5,001		5,001		5,001
Non-cumulative perpetual preferred stock	$ 500	$ 500		$ 500
Series A, liquidation preference per share	$ 100,000
Treasury stock, approximate aggregate cost		450
Stock repurchase program, remaining purchase authorization amount				225
Common stock purchase program, authorized amount	675
Common shares available for purchase						13,250,000
Treasury stock purchased		10,700,000
Average historical cost per share			$ 42.06
Realized (losses) gains from sales of investment securities				81	286
Unrealized pre-tax gains (losses) included in other comprehensive income						65	102
Deferred taxes related to unrealized pre-tax gains						$ 26	$ 41
State Street Capital Trust III [Member]
Subordinated borrowing, interest rate	6.001%
Junior subordinated debentures, due date	2042