Loans, Allowance for Loan Losses and Allowance for Unfunded Credit Commitments - Allowance for Credit Losses: Unfunded Credit Commitments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	$ 396	$ 590	$ 448	$ 305
Provision for credit losses (1)	(9)	(54)	41	247
Allowance for credit loss, ending balance	398	513	398	513
Increase in allowance for credit loss	2
Cumulative adjustment for the adoption of amendment
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance				25
Unfunded Credit Commitments
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	120	99	121	67
Provision for credit losses (1)	29	2	28	11
Allowance for credit loss, ending balance	149	101	149	101
Increase in allowance for credit loss	29
Unfunded Credit Commitments | Boston Private
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses (1)	2		2
Unfunded Credit Commitments | Cumulative adjustment for the adoption of amendment
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	$ 0	$ 0	$ 0	$ 23