Loans and Allowance for Credit Losses: Loans and Unfunded Credit Commitments - Activity in Allowance for Unfunded Commitments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Increase (decrease) in allowance for credit loss	$ 26
Allowance for credit loss, beginning balance	448	$ 305	$ 282
Provision (Reduction) for Loans	66	189	94
Allowance for credit loss, ending balance	422	448	305
Initial Allowance on PCD Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	22	25
Allowance for credit loss, ending balance		22	25
Unfunded credit commitments
Financing Receivable, Allowance for Credit Loss [Line Items]
Increase (decrease) in allowance for credit loss	50
Allowance for credit loss, beginning balance	121	68	55
Provision (Reduction) for Loans	50	30	13
Allowance for credit loss, ending balance	171	121	68
Unfunded credit commitments | Boston Private
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision (Reduction) for Loans	2
Unfunded credit commitments | Initial Allowance on PCD Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	$ 0	23	0
Allowance for credit loss, ending balance		$ 0	$ 23