Regulatory Capital (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained earnings	$ 20,387	$ 18,856
Capital ratio: required common equity tier 1 capital	7.50%	6.50%
Capital ratio: required tier 1 capital	9.00%	8.00%
Capital ratio: required total capital	11.00%	10.00%
Capital ratio: required tier 1 leverage	4.00%	4.00%
Basel III Advanced Approaches
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 10,922	$ 10,302
Retained earnings	20,387	18,856
Accumulated other comprehensive income (loss)	(1,661)	(972)
Treasury stock, at cost	(8,735)	(9,029)
Total	20,913	19,157
Goodwill and other intangible assets, net of associated deferred tax liabilities	(7,016)	(6,877)
Other adjustments(4)	(194)	(76)
Common equity tier 1 capital	13,703	12,204
Preferred stock	3,690	3,196
Trust preferred capital securities subject to phase-out from tier 1 capital	0	0
Other adjustments	0	(18)
Tier 1 capital	17,393	15,382
Qualifying subordinated long-term debt	761	980
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	5	4
Other adjustments	0	1
Total capital	18,159	16,367
Credit risk(5)	50,272	52,000
Operational risk(6)	45,840	45,822
Market risk	1,255	1,334
Total RWAs	97,367	99,156
Adjusted quarterly average assets	$ 214,103	$ 209,328
Common equity tier 1 capital	14.10%	12.30%
Tier 1 capital	17.90%	15.50%
Total capital	18.70%	16.50%
Tier 1 leveraged	8.10%	7.30%
Basel III Standardized Approach
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 10,922	$ 10,302
Retained earnings	20,387	18,856
Accumulated other comprehensive income (loss)	(1,661)	(972)
Treasury stock, at cost	(8,735)	(9,029)
Total	20,913	19,157
Goodwill and other intangible assets, net of associated deferred tax liabilities	(7,016)	(6,877)
Other adjustments(4)	(194)	(76)
Common equity tier 1 capital	13,703	12,204
Preferred stock	3,690	3,196
Trust preferred capital securities subject to phase-out from tier 1 capital	0	0
Other adjustments	0	(18)
Tier 1 capital	17,393	15,382
Qualifying subordinated long-term debt	761	980
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	74	72
Other adjustments	0	1
Total capital	18,228	16,435
Credit risk(5)	104,515	101,349
Market risk	1,255	1,334
Total RWAs	105,770	102,683
Adjusted quarterly average assets	$ 214,103	$ 209,328
Common equity tier 1 capital	13.00%	11.90%
Tier 1 capital	16.40%	15.00%
Total capital	17.20%	16.00%
Tier 1 leveraged	8.10%	7.30%
State Street Bank | Basel III Advanced Approaches
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 13,262	$ 11,612
Retained earnings	13,969	12,312
Accumulated other comprehensive income (loss)	(1,450)	(809)
Treasury stock, at cost	0	0
Total	25,781	23,115
Goodwill and other intangible assets, net of associated deferred tax liabilities	(6,721)	(6,579)
Other adjustments(4)	(48)	(5)
Common equity tier 1 capital	19,012	16,531
Preferred stock	0	0
Trust preferred capital securities subject to phase-out from tier 1 capital	0	0
Other adjustments	0	0
Tier 1 capital	19,012	16,531
Qualifying subordinated long-term debt	759	983
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	0	0
Other adjustments	0	0
Total capital	19,771	17,514
Credit risk(5)	47,732	49,489
Operational risk(6)	45,315	45,295
Market risk	1,255	1,334
Total RWAs	94,302	96,118
Adjusted quarterly average assets	$ 211,477	$ 206,070
Common equity tier 1 capital	20.20%	17.20%
Tier 1 capital	20.20%	17.20%
Total capital	21.00%	18.20%
Tier 1 leveraged	9.00%	8.00%
State Street Bank | Basel III Standardized Approach
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 13,262	$ 11,612
Retained earnings	13,969	12,312
Accumulated other comprehensive income (loss)	(1,450)	(809)
Treasury stock, at cost	0	0
Total	25,781	23,115
Goodwill and other intangible assets, net of associated deferred tax liabilities	(6,721)	(6,579)
Other adjustments(4)	(48)	(5)
Common equity tier 1 capital	19,012	16,531
Preferred stock	0	0
Trust preferred capital securities subject to phase-out from tier 1 capital	0	0
Other adjustments	0	0
Tier 1 capital	19,012	16,531
Qualifying subordinated long-term debt	759	983
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	75	72
Other adjustments	0	0
Total capital	19,846	17,586
Credit risk(5)	101,444	98,433
Market risk	1,255	1,334
Total RWAs	102,699	99,767
Adjusted quarterly average assets	$ 211,477	$ 206,070
Common equity tier 1 capital	18.50%	16.60%
Tier 1 capital	18.50%	16.60%
Total capital	19.30%	17.60%
Tier 1 leveraged	9.00%	8.00%