Long-Term Debt (Narrative) (Details) (USD $)	1 Months Ended		12 Months Ended
	Dec. 31, 2012		Dec. 31, 2012		Dec. 31, 2011
Debt Instrument [Line Items]
Bank Notes Authorized			$ 5,000,000,000
Senior Debt Securities Authorized Under Board Authority	4,000,000,000		4,000,000,000
Subordinated Debt Authorized	1,500,000,000		1,500,000,000
Long-term debt	7,429,000,000		7,429,000,000		8,131,000,000
Capital lease included in long term debt	576,000,000		576,000,000		565,000,000
One Lincoln Street Headquarters Building and Parking Garage
Debt Instrument [Line Items]
Capital lease included in long term debt	387,000,000		387,000,000		410,000,000
Office Building in the UK
Debt Instrument [Line Items]
Capital lease included in long term debt	269,000,000		269,000,000		265,000,000
Construction in Progress on New Building and Premises and Equipment
Debt Instrument [Line Items]
Capital lease included in long term debt	50,000,000		50,000,000		18,000,000
State Street Capital Trusts I And IV
Debt Instrument [Line Items]
Number of statutory business trusts			2
Issued trust preferred capital securities			955,000,000,000
Number of years interest payments received on debentures deferral			5 years
2.15% notes due 2012
Debt Instrument [Line Items]
Long-term debt	0		0		1,500,000,000
2.875% notes due 2016(1)
Debt Instrument [Line Items]
Long-term debt	1,014,000,000	[1]	1,014,000,000	[1]	999,000,000	[1]
Senior notes interest rate percentage	2.875%		2.875%
4.375% notes due 2021(1)
Debt Instrument [Line Items]
Long-term debt	780,000,000	[1]	780,000,000	[1]	757,000,000	[1]
Senior notes interest rate percentage	4.375%		4.375%
4.30% notes due 2014
Debt Instrument [Line Items]
Long-term debt	507,000,000		507,000,000		512,000,000
Subordinated borrowing interest rate			4.30%
Subordinated borrowing, due date			May 30, 2014
5.375% notes due 2017
Debt Instrument [Line Items]
Long-term debt	450,000,000		450,000,000		450,000,000
Subordinated borrowing interest rate			5.375%
Subordinated borrowing, due date			Apr. 30, 2017
Floating-rate notes due 2014
Debt Instrument [Line Items]
Long-term debt	250,000,000		250,000,000		250,000,000
Subordinated borrowing, due date			Mar. 07, 2014
Floating-rate notes due 2012
Debt Instrument [Line Items]
Long-term debt	0		0		250,000,000
7.35% notes due 2026
Debt Instrument [Line Items]
Long-term debt	150,000,000		150,000,000		150,000,000
Subordinated borrowing interest rate			7.35%
Subordinated borrowing, due date			Jun. 15, 2026
Extendible notes due 2014
Debt Instrument [Line Items]
Long-term debt	1,000,000,000		1,000,000,000		0
Long Term Debt, Duration	13 months
Subordinated borrowing, due date			Jan. 13, 2014
Date Maturity Extension Option Begins			Jan. 18, 2013
Maximum Maturity Date for Extension Option			Jan. 15, 2016
Date at Which Optional Redemptions of Debt are Allowable			Jan. 15, 2015
Percent of Principal Amount Redeemed Under Redemption Option			100.00%
5.25% subordinated notes due 2018(1)
Debt Instrument [Line Items]
Long-term debt	453,000,000	[1]	453,000,000	[1]	453,000,000	[1]
Subordinated borrowing interest rate			5.25%
5.30% subordinated notes due 2016
Debt Instrument [Line Items]
Long-term debt	$ 414,000,000		$ 414,000,000		$ 419,000,000
Subordinated borrowing interest rate			5.30%
4.956% Percent Junior Subordinated Debenture Due 2018
Debt Instrument [Line Items]
Subordinated borrowing interest rate			4.956%
Subordinated borrowing, due date			Mar. 15, 2018

[1] We have entered into interest-rate swap agreements, recorded as fair value hedges, to modify our interest expense on these senior and subordinated notes from a fixed rate to a floating rate. As of December 31, 2012 and 2011, we recorded an increase of $174 million and $140 million, respectively, in the carrying value of long-term debt associated with fair value hedges. Refer to note 16 for additional information about derivatives.