Regulatory Capital (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained earnings	$ 23,442	$ 21,918
Capital ratio: required common equity tier 1 capital	8.00%	8.50%
Capital ratio: required tier 1 capital	0.095	0.100
Capital ratio: required total capital	0.115	0.120
Tier 1 leverage capital ratio, minimum	0.040	0.040
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	$ 10,709	$ 10,636
Retained earnings	23,442	21,918
Accumulated other comprehensive income (loss)	187	(870)
Treasury stock, at cost	(10,609)	(10,209)
Total	23,729	21,475
Goodwill and other intangible assets, net of associated deferred tax liabilities	(9,019)	(9,112)
Other adjustments(1)	(333)	(150)
Common equity tier 1 capital	14,377	12,213
Preferred stock	2,471	2,962
Tier 1 capital	16,848	15,175
Qualifying subordinated long-term debt	961	1,095
Allowance for credit losses	1	5
Total capital	17,810	16,275
Credit risk	63,367	54,763
Operational risk	44,150	47,963
Market risk	2,188	1,638
Total risk-weighted assets	109,705	104,364
Adjusted quarterly average assets	$ 263,490	$ 219,624
Common equity tier 1 capital	13.10%	11.70%
Tier 1 capital	0.154	0.145
Total capital	0.162	0.156
Tier 1 leverage capital ratio, actual	0.064	0.069
Basel III Standardized Approach
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 10,709	$ 10,636
Retained earnings	23,442	21,918
Accumulated other comprehensive income (loss)	187	(870)
Treasury stock, at cost	(10,609)	(10,209)
Total	23,729	21,475
Goodwill and other intangible assets, net of associated deferred tax liabilities	(9,019)	(9,112)
Other adjustments(1)	(333)	(150)
Common equity tier 1 capital	14,377	12,213
Preferred stock	2,471	2,962
Tier 1 capital	16,848	15,175
Qualifying subordinated long-term debt	961	1,095
Allowance for credit losses	148	90
Total capital	17,957	16,360
Credit risk	114,892	102,367
Market risk	2,188	1,638
Total risk-weighted assets	117,080	104,005
Adjusted quarterly average assets	$ 263,490	$ 219,624
Common equity tier 1 capital	12.30%	11.70%
Tier 1 capital	0.144	0.146
Total capital	0.153	0.157
Tier 1 leverage capital ratio, actual	0.064	0.069
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	12,939	13,218
Accumulated other comprehensive income (loss)	371	(654)
Treasury stock, at cost	0	0
Total	26,203	25,457
Goodwill and other intangible assets, net of associated deferred tax liabilities	(8,745)	(8,839)
Other adjustments(1)	(152)	(1)
Common equity tier 1 capital	17,306	16,617
Preferred stock	0	0
Tier 1 capital	17,306	16,617
Qualifying subordinated long-term debt	966	1,099
Allowance for credit losses	10	3
Total capital	18,282	17,719
Credit risk	58,960	51,610
Operational risk	43,663	44,138
Market risk	2,188	1,638
Total risk-weighted assets	104,811	97,386
Adjusted quarterly average assets	$ 260,489	$ 216,397
Common equity tier 1 capital	16.50%	17.10%
Tier 1 capital	0.165	0.171
Total capital	0.174	0.182
Tier 1 leverage capital ratio, actual	0.066	0.077
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	12,939	13,218
Accumulated other comprehensive income (loss)	371	(654)
Treasury stock, at cost	0	0
Total	26,203	25,457
Goodwill and other intangible assets, net of associated deferred tax liabilities	(8,745)	(8,839)
Other adjustments(1)	(152)	(1)
Common equity tier 1 capital	17,306	16,617
Preferred stock	0	0
Tier 1 capital	17,306	16,617
Qualifying subordinated long-term debt	966	1,099
Allowance for credit losses	148	90
Total capital	18,420	17,806
Credit risk	110,797	98,979
Market risk	2,188	1,638
Total risk-weighted assets	112,985	100,617
Adjusted quarterly average assets	$ 260,489	$ 216,397
Common equity tier 1 capital	15.30%	16.50%
Tier 1 capital	0.153	0.165
Total capital	0.163	0.177
Tier 1 leverage capital ratio, actual	0.066	0.077