Short-Term Borrowings and Long-Term Debt - Long-term Debt (Details)					1 Months Ended	12 Months Ended
	Oct. 28, 2021 USD ($)	May 13, 2021 USD ($)	Feb. 02, 2021 USD ($)	Jun. 05, 2020 USD ($)	Jan. 31, 2015 USD ($)	Dec. 31, 2021 USD ($) quarterly_interest_payments	Dec. 31, 2020 USD ($)
Debt Instrument [Line Items]
Long-term debt						$ 2,570,000,000	$ 844,000,000
Discount on debt (less than $1 million for all but 1.800% Senior Notes due 2031)						1,000,000
Senior Notes | 3.50% Senior Notes
Debt Instrument [Line Items]
Principal value					$ 350,000,000	$ 350,000,000
Stated interest rate					3.50%	3.50%
Proceeds from issuance of senior notes, net of discount and issuance cost					$ 346,000,000
Long-term debt						$ 349,000,000	349,000,000
Debt issuance costs						1,000,000
Discount on debt (less than $1 million for all but 1.800% Senior Notes due 2031)						1,000,000
Senior Notes | 3.125% Senior Notes
Debt Instrument [Line Items]
Principal value				$ 500,000,000		$ 500,000,000
Stated interest rate				3.125%		3.125%
Proceeds from issuance of senior notes, net of discount and issuance cost				$ 496,000,000
Long-term debt						$ 496,000,000	495,000,000
Debt issuance costs						4,000,000
Discount on debt (less than $1 million for all but 1.800% Senior Notes due 2031)						1,000,000
Senior Notes | 1.800% Senior Notes
Debt Instrument [Line Items]
Principal value			$ 500,000,000			$ 500,000,000
Stated interest rate			1.80%			1.80%
Proceeds from issuance of senior notes, net of discount and issuance cost			$ 494,000,000
Long-term debt						$ 494,000,000	0
Debt issuance costs						4,000,000
Discount on debt (less than $1 million for all but 1.800% Senior Notes due 2031)						2,000,000
Senior Notes | 2.100% Senior Notes
Debt Instrument [Line Items]
Principal value		$ 500,000,000				$ 500,000,000
Stated interest rate		2.10%				2.10%
Proceeds from issuance of senior notes, net of discount and issuance cost		$ 496,000,000
Long-term debt						$ 496,000,000	0
Debt issuance costs						3,000,000
Senior Notes | 1.800% Senior Notes
Debt Instrument [Line Items]
Principal value	$ 650,000,000					$ 650,000,000
Stated interest rate	1.80%					1.80%
Proceeds from issuance of senior notes, net of discount and issuance cost	$ 644,000,000
Long-term debt						$ 645,000,000	0
Debt issuance costs						4,000,000
Junior subordinated debentures
Debt Instrument [Line Items]
Principal value						100,000,000
Long-term debt						90,000,000	$ 0
Junior subordinated debentures | Boston Private Capital Trust II Junior Subordinated Debentures (Trust II)
Debt Instrument [Line Items]
Principal value						$ 100,000,000
Basis spread on variable rate						1.68%
Number of quarterly interest payments allowed to defer | quarterly_interest_payments						20
Effective percentage on debt						1.898%
Investment in trust, carrying value						$ 3,000,000
Junior subordinated debentures | Boston Private Capital Trust I Junior Subordinated Debenture (Trust I)
Debt Instrument [Line Items]
Principal value						$ 1,000,000
Stated interest rate	4.875%
Effective percentage on debt						4.875%
Investment in trust, carrying value						$ 3,000,000