Regulatory Capital (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained earnings	$ 23,751	$ 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	0.05
Basel III Advanced Approaches
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 10,731	$ 10,709
Retained earnings	23,751	23,442
Accumulated other comprehensive income (loss)	(418)	187
Treasury stock, at cost	(11,035)	(10,609)
Total	23,029	23,729
Goodwill and other intangible assets, net of associated deferred tax liabilities	(9,149)	(9,019)
Other adjustments(1)	(437)	(333)
Common equity tier 1 capital	13,443	14,377
Preferred stock	1,976	2,471
Tier 1 capital	15,419	16,848
Qualifying subordinated long-term debt	1,801	961
Allowance for credit losses	0	1
Total capital	17,220	17,810
Credit risk	66,726	63,367
Operational risk	44,075	44,150
Market risk	2,250	2,188
Total risk-weighted assets	113,051	109,705
Adjusted quarterly average assets	$ 285,480	$ 263,490
Common equity tier 1 capital	11.90%	13.10%
Tier 1 capital	0.136	0.154
Total capital	0.152	0.162
Tier 1 leverage capital ratio, actual	0.054	0.064
Basel III Standardized Approach
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 10,731	$ 10,709
Retained earnings	23,751	23,442
Accumulated other comprehensive income (loss)	(418)	187
Treasury stock, at cost	(11,035)	(10,609)
Total	23,029	23,729
Goodwill and other intangible assets, net of associated deferred tax liabilities	(9,149)	(9,019)
Other adjustments(1)	(437)	(333)
Common equity tier 1 capital	13,443	14,377
Preferred stock	1,976	2,471
Tier 1 capital	15,419	16,848
Qualifying subordinated long-term debt	1,801	961
Allowance for credit losses	135	148
Total capital	17,355	17,957
Credit risk	122,074	114,892
Market risk	2,250	2,188
Total risk-weighted assets	124,324	117,080
Adjusted quarterly average assets	$ 285,480	$ 263,490
Common equity tier 1 capital	10.80%	12.30%
Tier 1 capital	0.124	0.144
Total capital	0.140	0.153
Tier 1 leverage capital ratio, actual	0.054	0.064
State Street Bank | Basel III Advanced Approaches
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 12,893	$ 12,893
Retained earnings	13,479	12,939
Accumulated other comprehensive income (loss)	(221)	371
Treasury stock, at cost	0	0
Total	26,151	26,203
Goodwill and other intangible assets, net of associated deferred tax liabilities	(8,880)	(8,745)
Other adjustments(1)	(250)	(152)
Common equity tier 1 capital	17,021	17,306
Preferred stock	0	0
Tier 1 capital	17,021	17,306
Qualifying subordinated long-term debt	963	966
Allowance for credit losses	0	10
Total capital	17,984	18,282
Credit risk	62,082	58,960
Operational risk	43,600	43,663
Market risk	2,250	2,188
Total risk-weighted assets	107,932	104,811
Adjusted quarterly average assets	$ 282,319	$ 260,489
Common equity tier 1 capital	15.80%	16.50%
Tier 1 capital	0.158	0.165
Total capital	0.167	0.174
Tier 1 leverage capital ratio, actual	0.060	0.066
State Street Bank | Basel III Standardized Approach
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 12,893	$ 12,893
Retained earnings	13,479	12,939
Accumulated other comprehensive income (loss)	(221)	371
Treasury stock, at cost	0	0
Total	26,151	26,203
Goodwill and other intangible assets, net of associated deferred tax liabilities	(8,880)	(8,745)
Other adjustments(1)	(250)	(152)
Common equity tier 1 capital	17,021	17,306
Preferred stock	0	0
Tier 1 capital	17,021	17,306
Qualifying subordinated long-term debt	963	966
Allowance for credit losses	135	148
Total capital	18,119	18,420
Credit risk	119,020	110,797
Market risk	2,250	2,188
Total risk-weighted assets	121,270	112,985
Adjusted quarterly average assets	$ 282,319	$ 260,489
Common equity tier 1 capital	14.00%	15.30%
Tier 1 capital	0.140	0.153
Total capital	0.149	0.163
Tier 1 leverage capital ratio, actual	0.060	0.066