Regulatory Capital (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained earnings	$ 18,856	$ 17,459
Capital ratio: required common equity tier 1 capital	6.50%	5.50%
Capital ratio: required tier 1 capital	8.00%	7.00%
Capital ratio: required total capital	10.00%	9.00%
Capital ratio: required tier 1 leverage	4.00%	4.00%
Percentage of goodwill and other intangible assets net of associated deferred tax liabilities allocated to other intangible assets	80.00%	60.00%
Basel III Advanced Approaches
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 10,302	$ 10,286
Retained earnings	18,856	17,459
Accumulated other comprehensive income (loss)	(972)	(1,936)
Treasury stock, at cost	(9,029)	(7,682)
Total	19,157	18,127
Goodwill and other intangible assets, net of associated deferred tax liabilities	(6,877)	(6,348)
Other adjustments	(76)	(155)
Common equity tier 1 capital	12,204	11,624
Preferred stock	3,196	3,196
Trust preferred capital securities subject to phase-out from tier 1 capital	0	0
Other adjustments	(18)	(103)
Tier 1 capital	15,382	14,717
Qualifying subordinated long-term debt	980	1,172
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	4	19
Other adjustments	1	1
Total capital	16,367	15,909
Credit risk	49,976	50,900
Operational risk(4)	45,822	44,579
Market risk	3,358	3,822
Total risk-weighted assets	99,156	99,301
Adjusted quarterly average assets	$ 209,328	$ 226,310
Common equity tier 1 capital	12.30%	11.70%
Tier 1 capital	15.50%	14.80%
Total capital	16.50%	16.00%
Tier 1 leveraged	7.30%	6.50%
Basel III Standardized Approach
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 10,302	$ 10,286
Retained earnings	18,856	17,459
Accumulated other comprehensive income (loss)	(972)	(1,936)
Treasury stock, at cost	(9,029)	(7,682)
Total	19,157	18,127
Goodwill and other intangible assets, net of associated deferred tax liabilities	(6,877)	(6,348)
Other adjustments	(76)	(155)
Common equity tier 1 capital	12,204	11,624
Preferred stock	3,196	3,196
Trust preferred capital securities subject to phase-out from tier 1 capital	0	0
Other adjustments	(18)	(103)
Tier 1 capital	15,382	14,717
Qualifying subordinated long-term debt	980	1,172
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	72	77
Other adjustments	1	1
Total capital	16,435	15,967
Credit risk	101,349	98,125
Market risk	1,334	1,751
Total risk-weighted assets	102,683	99,876
Adjusted quarterly average assets	$ 209,328	$ 226,310
Common equity tier 1 capital	11.90%	11.60%
Tier 1 capital	15.00%	14.70%
Total capital	16.00%	16.00%
Tier 1 leveraged	7.30%	6.50%
State Street Bank | Basel III Advanced Approaches
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 11,612	$ 11,376
Retained earnings	12,312	12,285
Accumulated other comprehensive income (loss)	(809)	(1,648)
Treasury stock, at cost	0	0
Total	23,115	22,013
Goodwill and other intangible assets, net of associated deferred tax liabilities	(6,579)	(6,060)
Other adjustments	(5)	(148)
Common equity tier 1 capital	16,531	15,805
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	16,531	15,805
Qualifying subordinated long-term debt	983	1,179
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	0	15
Other adjustments	0	0
Total capital	17,514	16,999
Credit risk	47,448	47,383
Operational risk(4)	45,295	44,043
Market risk	3,375	3,822
Total risk-weighted assets	96,118	95,248
Adjusted quarterly average assets	$ 206,070	$ 222,584
Common equity tier 1 capital	17.20%	16.60%
Tier 1 capital	17.20%	16.60%
Total capital	18.20%	17.80%
Tier 1 leveraged	8.00%	7.10%
State Street Bank | Basel III Standardized Approach
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 11,612	$ 11,376
Retained earnings	12,312	12,285
Accumulated other comprehensive income (loss)	(809)	(1,648)
Treasury stock, at cost	0	0
Total	23,115	22,013
Goodwill and other intangible assets, net of associated deferred tax liabilities	(6,579)	(6,060)
Other adjustments	(5)	(148)
Common equity tier 1 capital	16,531	15,805
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	16,531	15,805
Qualifying subordinated long-term debt	983	1,179
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	72	77
Other adjustments	0	0
Total capital	17,586	17,061
Credit risk	98,433	94,413
Market risk	1,334	1,751
Total risk-weighted assets	99,767	96,164
Adjusted quarterly average assets	$ 206,070	$ 222,584
Common equity tier 1 capital	16.60%	16.40%
Tier 1 capital	16.60%	16.40%
Total capital	17.60%	17.70%
Tier 1 leveraged	8.00%	7.10%