Regulatory Capital (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained earnings	$ 19,856	$ 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	$ 10,324	$ 10,302
Retained earnings	19,856	18,856
Accumulated other comprehensive income (loss)	(1,446)	(972)
Treasury stock, at cost	(9,317)	(9,029)
Total	19,417	19,157
Goodwill and other intangible assets, net of associated deferred tax liabilities	(7,008)	(6,877)
Other adjustments	(186)	(76)
Common equity tier 1 capital	12,223	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	15,419	15,382
Qualifying subordinated long-term debt	765	980
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	0	4
Other adjustments	0	1
Total capital	16,184	16,367
Credit risk	48,308	49,976
Operational risk(4)	45,991	45,822
Market risk	4,203	3,358
Total risk-weighted assets	98,502	99,156
Adjusted quarterly average assets	$ 216,896	$ 209,328
Common equity tier 1 capital	12.40%	12.30%
Tier 1 capital	15.70%	15.50%
Total capital	16.40%	16.50%
Tier 1 leveraged	7.10%	7.30%
Basel III Standardized Approach
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 10,324	$ 10,302
Retained earnings	19,856	18,856
Accumulated other comprehensive income (loss)	(1,446)	(972)
Treasury stock, at cost	(9,317)	(9,029)
Total	19,417	19,157
Goodwill and other intangible assets, net of associated deferred tax liabilities	(7,008)	(6,877)
Other adjustments	(186)	(76)
Common equity tier 1 capital	12,223	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	15,419	15,382
Qualifying subordinated long-term debt	765	980
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	73	72
Other adjustments	0	1
Total capital	16,257	16,435
Credit risk	106,063	101,349
Market risk	1,677	1,334
Total risk-weighted assets	107,740	102,683
Adjusted quarterly average assets	$ 216,896	$ 209,328
Common equity tier 1 capital	11.30%	11.90%
Tier 1 capital	14.30%	15.00%
Total capital	15.10%	16.00%
Tier 1 leveraged	7.10%	7.30%
State Street Bank | Basel III Advanced Approaches
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common stock and related surplus	$ 11,612	$ 11,612
Retained earnings	13,189	12,312
Accumulated other comprehensive income (loss)	(1,251)	(809)
Treasury stock, at cost	0	0
Total	23,550	23,115
Goodwill and other intangible assets, net of associated deferred tax liabilities	(6,711)	(6,579)
Other adjustments	(44)	(5)
Common equity tier 1 capital	16,795	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	16,795	16,531
Qualifying subordinated long-term debt	765	983
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	0	0
Other adjustments	0	0
Total capital	17,560	17,514
Credit risk	45,689	47,448
Operational risk(4)	45,423	45,295
Market risk	4,205	3,375
Total risk-weighted assets	95,317	96,118
Adjusted quarterly average assets	$ 214,670	$ 206,070
Common equity tier 1 capital	17.60%	17.20%
Tier 1 capital	17.60%	17.20%
Total capital	18.40%	18.20%
Tier 1 leveraged	7.80%	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	13,189	12,312
Accumulated other comprehensive income (loss)	(1,251)	(809)
Treasury stock, at cost	0	0
Total	23,550	23,115
Goodwill and other intangible assets, net of associated deferred tax liabilities	(6,711)	(6,579)
Other adjustments	(44)	(5)
Common equity tier 1 capital	16,795	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	16,795	16,531
Qualifying subordinated long-term debt	765	983
Trust preferred capital securities phased out of tier 1 capital	0	0
ALLL and other	73	72
Other adjustments	0	0
Total capital	17,633	17,586
Credit risk	103,156	98,433
Market risk	1,677	1,334
Total risk-weighted assets	104,833	99,767
Adjusted quarterly average assets	$ 214,670	$ 206,070
Common equity tier 1 capital	16.00%	16.60%
Tier 1 capital	16.00%	16.60%
Total capital	16.80%	17.60%
Tier 1 leveraged	7.80%	8.00%