Regulatory Capital (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained earnings	$ 24,785	$ 23,442
Capital ratio: required common equity tier 1 capital	8.00%	8.00%
Capital ratio: required tier 1 capital	0.095	0.095
Capital ratio: required total capital	0.115	0.115
Tier 1 leverage capital ratio, minimum	0.040	0.040
Capital conservation buffer	0.025
Stress capital buffer	2.50%
Global systemically important bank	0.010
Countercyclical capital buffer	0
Leverage ratio minimum	0.050
Basel III Advanced Approaches
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 11,267	$ 10,709
Retained earnings	24,785	23,442
Accumulated other comprehensive income (loss)		187
Treasury stock, at cost	(10,032)	(10,609)
Total	25,338	23,729
Goodwill and other intangible assets, net of associated deferred tax liabilities	(9,040)	(9,019)
Other adjustments	(467)	(333)
Common equity tier 1 capital	15,831	14,377
Preferred stock	1,976	2,471
Tier 1 capital	17,807	16,848
Qualifying subordinated long-term debt	1,590	961
Allowance for credit losses	0	1
Total capital	19,397	17,810
Credit risk	66,503	63,367
Operational risk	45,750	44,150
Market risk	2,625	2,188
Total risk-weighted assets	114,878	109,705
Adjusted quarterly average assets	$ 281,952	$ 263,490
Common equity tier 1 capital	13.80%	13.10%
Tier 1 capital	0.155	0.154
Total capital	0.169	0.162
Tier 1 leverage capital ratio, actual	0.063	0.064
Basel III Standardized Approach
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 11,267	$ 10,709
Retained earnings	24,785	23,442
Accumulated other comprehensive income (loss)	(682)	187
Treasury stock, at cost	(10,032)	(10,609)
Total	25,338	23,729
Goodwill and other intangible assets, net of associated deferred tax liabilities	(9,040)	(9,019)
Other adjustments	(467)	(333)
Common equity tier 1 capital	15,831	14,377
Preferred stock	1,976	2,471
Tier 1 capital	17,807	16,848
Qualifying subordinated long-term debt	1,590	961
Allowance for credit losses	117	148
Total capital	19,514	17,957
Credit risk	114,604	114,892
Market risk	2,625	2,188
Total risk-weighted assets	117,229	117,080
Adjusted quarterly average assets	$ 281,952	$ 263,490
Common equity tier 1 capital	13.50%	12.30%
Tier 1 capital	0.152	0.144
Total capital	0.166	0.153
Tier 1 leverage capital ratio, actual	0.063	0.064
State Street Bank | Basel III Advanced Approaches
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 12,887	$ 12,893
Retained earnings	14,919	12,939
Accumulated other comprehensive income (loss)	(465)	371
Treasury stock, at cost	0	0
Total	27,341	26,203
Goodwill and other intangible assets, net of associated deferred tax liabilities	(8,770)	(8,745)
Other adjustments	(281)	(152)
Common equity tier 1 capital	18,290	17,306
Preferred stock	0	0
Tier 1 capital	18,290	17,306
Qualifying subordinated long-term debt	754	966
Allowance for credit losses	0	10
Total capital	19,044	18,282
Credit risk	59,635	58,960
Operational risk	43,113	43,663
Market risk	2,625	2,188
Total risk-weighted assets	105,373	104,811
Adjusted quarterly average assets	$ 278,705	$ 260,489
Common equity tier 1 capital	17.40%	16.50%
Tier 1 capital	0.174	0.165
Total capital	0.181	0.174
Tier 1 leverage capital ratio, actual	0.066	0.066
State Street Bank | Basel III Standardized Approach
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 12,887	$ 12,893
Retained earnings	14,919	12,939
Accumulated other comprehensive income (loss)	(465)	371
Treasury stock, at cost	0	0
Total	27,341	26,203
Goodwill and other intangible assets, net of associated deferred tax liabilities	(8,770)	(8,745)
Other adjustments	(281)	(152)
Common equity tier 1 capital	18,290	17,306
Preferred stock	0	0
Tier 1 capital	18,290	17,306
Qualifying subordinated long-term debt	754	966
Allowance for credit losses	117	148
Total capital	19,161	18,420
Credit risk	111,464	110,797
Market risk	2,625	2,188
Total risk-weighted assets	114,089	112,985
Adjusted quarterly average assets	$ 278,705	$ 260,489
Common equity tier 1 capital	16.00%	15.30%
Tier 1 capital	0.160	0.153
Total capital	0.168	0.163
Tier 1 leverage capital ratio, actual	0.066	0.066