Regulatory Capital (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained earnings	$ 19,311	$ 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	$ 10,300	$ 10,302
Retained earnings	19,311	18,856
Accumulated other comprehensive income (loss)	(1,040)	(972)
Treasury stock, at cost	(9,334)	(9,029)
Total	19,237	19,157
Goodwill and other intangible assets, net of associated deferred tax liabilities	(7,169)	(6,877)
Other adjustments	(118)	(76)
Common equity tier 1 capital	11,950	12,204
Preferred stock	3,196	3,196
Trust preferred capital securities subject to phase-out from tier 1 capital	0	0
Other adjustments	0	(18)
Tier 1 capital	15,146	15,382
Qualifying subordinated long-term debt	961	980
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	0	4
Other adjustments	0	1
Total capital	16,107	16,367
Credit risk	48,843	49,976
Operational risk(4)	46,039	45,822
Market risk	3,630	3,358
Total risk-weighted assets	98,512	99,156
Adjusted quarterly average assets	$ 219,582	$ 209,328
Common equity tier 1 capital	12.10%	12.30%
Tier 1 capital	15.40%	15.50%
Total capital	16.40%	16.50%
Tier 1 leveraged	6.90%	7.30%
Basel III Standardized Approach
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 10,300	$ 10,302
Retained earnings	19,311	18,856
Accumulated other comprehensive income (loss)	(1,040)	(972)
Treasury stock, at cost	(9,334)	(9,029)
Total	19,237	19,157
Goodwill and other intangible assets, net of associated deferred tax liabilities	(7,169)	(6,877)
Other adjustments	(118)	(76)
Common equity tier 1 capital	11,950	12,204
Preferred stock	3,196	3,196
Trust preferred capital securities subject to phase-out from tier 1 capital	0	0
Other adjustments	0	(18)
Tier 1 capital	15,146	15,382
Qualifying subordinated long-term debt	961	980
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	72	72
Other adjustments	0	1
Total capital	16,179	16,435
Credit risk	108,946	101,349
Market risk	1,531	1,334
Total risk-weighted assets	110,477	102,683
Adjusted quarterly average assets	$ 219,582	$ 209,328
Common equity tier 1 capital	10.80%	11.90%
Tier 1 capital	13.70%	15.00%
Total capital	14.60%	16.00%
Tier 1 leveraged	6.90%	7.30%
State Street Bank | Basel III Advanced Approaches
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 11,612	$ 11,612
Retained earnings	12,442	12,312
Accumulated other comprehensive income (loss)	(898)	(809)
Treasury stock, at cost	0	0
Total	23,156	23,115
Goodwill and other intangible assets, net of associated deferred tax liabilities	(6,859)	(6,579)
Other adjustments	(1)	(5)
Common equity tier 1 capital	16,296	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	16,296	16,531
Qualifying subordinated long-term debt	962	983
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	0	0
Other adjustments	0	0
Total capital	17,258	17,514
Credit risk	46,164	47,448
Operational risk(4)	45,488	45,295
Market risk	3,632	3,375
Total risk-weighted assets	95,284	96,118
Adjusted quarterly average assets	$ 216,922	$ 206,070
Common equity tier 1 capital	17.10%	17.20%
Tier 1 capital	17.10%	17.20%
Total capital	18.10%	18.20%
Tier 1 leveraged	7.50%	8.00%
State Street Bank | Basel III Standardized Approach
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 11,612	$ 11,612
Retained earnings	12,442	12,312
Accumulated other comprehensive income (loss)	(898)	(809)
Treasury stock, at cost	0	0
Total	23,156	23,115
Goodwill and other intangible assets, net of associated deferred tax liabilities	(6,859)	(6,579)
Other adjustments	(1)	(5)
Common equity tier 1 capital	16,296	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	16,296	16,531
Qualifying subordinated long-term debt	962	983
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	72	72
Other adjustments	0	0
Total capital	17,330	17,586
Credit risk	106,132	98,433
Market risk	1,531	1,334
Total risk-weighted assets	107,663	99,767
Adjusted quarterly average assets	$ 216,922	$ 206,070
Common equity tier 1 capital	15.10%	16.60%
Tier 1 capital	15.10%	16.60%
Total capital	16.10%	17.60%
Tier 1 leveraged	7.50%	8.00%