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Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	6 Months Ended	9 Months Ended
	May 30, 2023	Jan. 31, 2026	Jun. 30, 2026	Sep. 30, 2025
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Stock Units, Granted			137,429
Fair value of RSUs/restricted stock granted to employees			$ 690
Reduction of shares due to performance measures			(24,996)
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of outstanding RSUs			$ 3,000
Stock price			$ 962
Unrecognized stock-based compensation expense			$ 1,500
Remaining weighted-average period			2 years
Restricted Stock Units And Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Stock Units, Granted		342,000
Award vesting period, years		3 years
Awards to employees cliff vesting		183,000
RSUs to employees that cliff vest, percentage		100.00%
RSUs to employees that cliff vest, date		Jan. 31, 2029
Performance Based Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Stock Units, Granted			137,429
Award vesting period, years		3 years
Awards to employees cliff vesting		137,000
RSUs to employees that cliff vest, percentage		100.00%
RSUs to employees that cliff vest, date		Jan. 31, 2029
Share based compensation vesting option exercised period			up to nine years
Intrinsic value of outstanding RSUs			$ 610
Stock price			$ 962
Unrecognized stock-based compensation expense			$ 354
Remaining weighted-average period			2 years
Reduction of shares due to performance measures		24,996	(24,996)
Fair value of RSUs/restricted stock granted to employees			$ 179
Performance Based Restricted Stock Units [Member] \| Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs to employees that cliff vest, percentage	25.00%
RSUs to employees that cliff vest, vesting date	May 30, 2027
Performance Based Restricted Stock Units [Member] \| Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs to employees that cliff vest, percentage	25.00%
RSUs to employees that cliff vest, vesting date	May 30, 2028
Performance Based Restricted Stock Units [Member] \| Share-based Payment Arrangement, Tranche Three [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs to employees that cliff vest, percentage	50.00%
RSUs to employees that cliff vest, vesting date	May 30, 2029
Performance Based Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation vesting option exercised period	up to nine years
Award vesting percentage of BlackRock's grant-date stock price	130.00%		125.00%
Award vesting trading period of BlackRock's grant-date stock price			20 days
Award vesting period of BlackRock's grant-date stock price	4 years		5 years
Award vesting period of BlackRock's performance period	3 years
Award performance measurement period				4 years
Aggregate intrinsic value of options exercised			$ 103
Unrecognized stock-based compensation expense			$ 57
Performance Based Stock Options [Member] \| Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service year of awards			2022
Performance Based Stock Options [Member] \| Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service year of awards			2023
Performance Based Stock Options [Member] \| Share-based Payment Arrangement, Tranche Three [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service year of awards			2024
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining weighted-average period			2 years 2 months 12 days
Time-based stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation vesting option exercised period	up to nine years
Unrecognized stock-based compensation expense			$ 57
Time-based stock options \| Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs to employees that cliff vest, percentage	25.00%
RSUs to employees that cliff vest, vesting date	May 30, 2027
Time-based stock options \| Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs to employees that cliff vest, percentage	25.00%
RSUs to employees that cliff vest, vesting date	May 30, 2028
Time-based stock options \| Share-based Payment Arrangement, Tranche Three [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs to employees that cliff vest, percentage	50.00%
RSUs to employees that cliff vest, vesting date	May 30, 2029