Long-Term Debt (Details) (USD $)	3 Months Ended	9 Months Ended
	Mar. 31, 2011	Sep. 30, 2011	Feb. 28, 2011
Senior notes issued	2,000,000,000
Non-cumulative perpetual preferred stock	500,000,000	500,000,000
Junior Subordinated Debentures [Member]
Subordinated borrowing interest rate	4.956%
Original junior subordinated debentures interest rate	6.001%
4.956% Junior Subordinated Debenture Due 2018 [Member]
Notes issued by parent company and non-banking subsidiaries, maturity date	Mar. 15, 2018	Mar. 15, 2018
Non-cumulative perpetual preferred stock		500,000,000
Issued junior subordinated debentures			500,000,000
Subordinated borrowing interest rate	4.956%	4.956%
8.25% Fixed-To-Floating-Rate Subordinated Notes Due To State Street Capital Trust III In 2042 [Member]
Subordinated borrowing interest rate	8.25%
2.875% Senior Notes [Member]
Senior notes issued	1,000,000,000
Notes issued by parent company and non-banking subsidiaries, maturity date	Mar. 07, 2016
Senior notes interest rate percentage	2.875%
4.375% Senior Notes [Member]
Senior notes issued	750,000,000
Notes issued by parent company and non-banking subsidiaries, maturity date	Mar. 07, 2021
Senior notes interest rate percentage	4.375%
Floating-Rate Senior Notes [Member]
Senior notes issued	250,000,000
Notes issued by parent company and non-banking subsidiaries, maturity date	Mar. 07, 2014
Series A Preferred Stock [Member]
Non-cumulative perpetual preferred stock	500,000,000
Preferred stock liquidation preference per share	100,000
State Street Capital Trust III [Member]
Notes issued by parent company and non-banking subsidiaries, maturity date	Dec. 31, 2042
Subordinated borrowing interest rate	6.001%