Regulatory Capital (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained earnings	$ 23,128	$ 21,918
Treasury stock, at cost	$ (10,626)	$ (10,209)
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
Basel III Advanced Approaches
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 10,696	$ 10,636
Retained earnings	23,128	21,918
Accumulated other comprehensive income (loss)	(111)	(870)
Treasury stock, at cost	(10,626)	(10,209)
Total	23,087	21,475
Goodwill and other intangible assets, net of associated deferred tax liabilities	(8,992)	(9,112)
Other adjustments	(270)	(150)
Common equity tier 1 capital	13,825	12,213
Preferred stock	2,471	2,962
Tier 1 capital	16,296	15,175
Qualifying subordinated long-term debt	964	1,095
Allowance for credit losses	30	5
Total capital	17,290	16,275
Credit risk	62,199	54,763
Operational risk	44,050	47,963
Securitizations	1,863	1,638
Equity	108,112	104,364
All other	$ 247,762	$ 219,624
Common equity tier 1 capital	12.80%	11.70%
Tier 1 capital	0.151	0.145
Total capital	0.160	0.156
Basel III Standardized Approach
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 10,696	$ 10,636
Retained earnings	23,128	21,918
Accumulated other comprehensive income (loss)	(111)	(870)
Treasury stock, at cost	(10,626)	(10,209)
Total	23,087	21,475
Goodwill and other intangible assets, net of associated deferred tax liabilities	(8,992)	(9,112)
Other adjustments	(270)	(150)
Common equity tier 1 capital	13,825	12,213
Preferred stock	2,471	2,962
Tier 1 capital	16,296	15,175
Qualifying subordinated long-term debt	964	1,095
Allowance for credit losses	153	90
Total capital	17,413	16,360
Credit risk	109,296	102,367
Securitizations	1,863	1,638
Equity	111,159	104,005
All other	$ 247,762	$ 219,624
Common equity tier 1 capital	12.40%	11.70%
Tier 1 capital	0.147	0.146
Total capital	0.157	0.157
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	13,404	13,218
Accumulated other comprehensive income (loss)	109	(654)
Treasury stock, at cost	0	0
Total	26,406	25,457
Goodwill and other intangible assets, net of associated deferred tax liabilities	(8,721)	(8,839)
Other adjustments	(107)	(1)
Common equity tier 1 capital	17,578	16,617
Preferred stock	0	0
Tier 1 capital	17,578	16,617
Qualifying subordinated long-term debt	969	1,099
Allowance for credit losses	39	3
Total capital	18,586	17,719
Credit risk	57,723	51,610
Operational risk	43,563	44,138
Securitizations	1,863	1,638
Equity	103,149	97,386
All other	$ 244,434	$ 216,397
Common equity tier 1 capital	17.00%	17.10%
Tier 1 capital	0.170	0.171
Total capital	0.180	0.182
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	13,404	13,218
Accumulated other comprehensive income (loss)	109	(654)
Treasury stock, at cost	0	0
Total	26,406	25,457
Goodwill and other intangible assets, net of associated deferred tax liabilities	(8,721)	(8,839)
Other adjustments	(107)	(1)
Common equity tier 1 capital	17,578	16,617
Preferred stock	0	0
Tier 1 capital	17,578	16,617
Qualifying subordinated long-term debt	969	1,099
Allowance for credit losses	153	90
Total capital	18,700	17,806
Credit risk	105,406	98,979
Securitizations	1,863	1,638
Equity	107,269	100,617
All other	$ 244,434	$ 216,397
Common equity tier 1 capital	16.40%	16.50%
Tier 1 capital	0.164	0.165
Total capital	0.174	0.177