Shareholders' Equity (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	9 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Shareholders' Equity [Line Items]
Non-cumulative perpetual preferred stock, shares	5,001	5,001	5,001
Non-cumulative perpetual preferred stock	$ 500	$ 500
Series A, liquidation preference per share	$ 100,000	$ 100,000
Common stock, approximate aggregate cost	675
Common stock purchased and held in trust, shares		406,000	420,000
Common stock purchase program, authorized amount		675
Common shares available for purchase			13,250,000
Common stock acquired,shares	16,300,000
Average historical cost per share	$ 41.38
Realized (losses) gains from sales of investment securities		140	(55)	368
Unrealized pre-tax gains (losses) included in other comprehensive income		76		(728)	46
Deferred taxes related to unrealized pre-tax gains			$ 30	$ 291	$ 18
Preferred stock dividend payment terms		three-month LIBOR plus 4.99%
Debt instrument, variable interest rate	4.99%	4.99%
State Street Capital Trust III [Member]
Shareholders' Equity [Line Items]
Subordinated borrowing, interest rate		6.001%
Junior subordinated debentures, due date		2042