Finance Receivable, Allowance for Credit Loss (Details) - USD ($) $ in Thousands			12 Months Ended
		Nov. 12, 2021	Dec. 31, 2021		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss			$ 146,922		$ 113,392	$ 67,740	$ 64,293
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method					(1,137)
Financing Receivable, Allowance for Credit Loss, Purchased with Credit Deterioration					1,157
Financing Receivable, Allowance for Credit Loss, Writeoff			(4,944)		(8,446)	(5,205)
Financing Receivable, Allowance for Credit Loss, Recovery			3,729		1,578	2,652
Financing Receivable, Credit Loss, Expense (Reversal)			18,205		52,500	6,000
Financing Receivable, Nonaccrual, No Allowance			621	[1]	35,528
Finance Receivable, Nonaccrual, with allowance			27,199	[1]	31,332
Financing Receivable, Nonaccrual	[1]		27,820		66,860
Blue Hills Bancorp, Inc. [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss			67,200
Financing Receivable, Credit Loss, Expense (Reversal)		$ 50,700
Commercial And Industrial [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss			14,402		21,086	17,594	15,760
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method					(1,984)
Financing Receivable, Allowance for Credit Loss, Purchased with Credit Deterioration					49
Financing Receivable, Allowance for Credit Loss, Writeoff			(3,474)		(2,309)	(244)
Financing Receivable, Allowance for Credit Loss, Recovery			2,686		289	1,131
Financing Receivable, Credit Loss, Expense (Reversal)			(6,062)		7,447	947
Financing Receivable, Nonaccrual, No Allowance			19		30,925
Finance Receivable, Nonaccrual, with allowance			3,420		3,804
Financing Receivable, Nonaccrual	[1]		3,439		34,729
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss			83,486		45,009	32,935	32,370
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method					(13,048)
Financing Receivable, Allowance for Credit Loss, Purchased with Credit Deterioration					337
Financing Receivable, Allowance for Credit Loss, Writeoff			0		(3,885)	(2,614)
Financing Receivable, Allowance for Credit Loss, Recovery			57		9	152
Financing Receivable, Credit Loss, Expense (Reversal)			24,023		28,661	3,027
Financing Receivable, Nonaccrual, No Allowance			0		0
Finance Receivable, Nonaccrual, with allowance			10,870		10,195
Financing Receivable, Nonaccrual	[1]		10,870		10,195
Construction Loans [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss			12,316		5,397	6,053	5,158
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method					(3,652)
Financing Receivable, Allowance for Credit Loss, Purchased with Credit Deterioration					0
Financing Receivable, Allowance for Credit Loss, Writeoff			0		0	0
Financing Receivable, Allowance for Credit Loss, Recovery			0		0	0
Financing Receivable, Credit Loss, Expense (Reversal)			5,900		2,996	895
Small Business [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss			3,508		5,095	1,746	1,756
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method					495
Financing Receivable, Allowance for Credit Loss, Purchased with Credit Deterioration					0
Financing Receivable, Allowance for Credit Loss, Writeoff			(219)		(380)	(509)
Financing Receivable, Allowance for Credit Loss, Recovery			98		33	122
Financing Receivable, Credit Loss, Expense (Reversal)			(1,466)		3,201	377
Financing Receivable, Nonaccrual, No Allowance					10
Finance Receivable, Nonaccrual, with allowance			44		815
Financing Receivable, Nonaccrual	[1]		44		825
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss			14,484		14,275	3,440	3,219
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method					9,828
Financing Receivable, Allowance for Credit Loss, Purchased with Credit Deterioration					423
Financing Receivable, Allowance for Credit Loss, Writeoff			0		(105)	0
Financing Receivable, Allowance for Credit Loss, Recovery			1		2	142
Financing Receivable, Credit Loss, Expense (Reversal)			(221)		687	79
Financing Receivable, Nonaccrual, No Allowance			602		4,593
Finance Receivable, Nonaccrual, with allowance			8,580		10,935
Financing Receivable, Nonaccrual	[1]		9,182		15,528
Home Equity Line of Credit [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss			17,986		22,060	5,576	5,608
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method					7,012
Financing Receivable, Allowance for Credit Loss, Purchased with Credit Deterioration					319
Financing Receivable, Allowance for Credit Loss, Writeoff			(69)		(142)	(240)
Financing Receivable, Allowance for Credit Loss, Recovery			249		210	318
Financing Receivable, Credit Loss, Expense (Reversal)			(4,417)		9,085	(110)
Financing Receivable, Nonaccrual, No Allowance			0		0
Finance Receivable, Nonaccrual, with allowance			3,781		5,427
Financing Receivable, Nonaccrual	[1]		3,781		5,427
Consumer Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Allowance for Credit Loss			740		470	396	$ 422
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method					212
Financing Receivable, Allowance for Credit Loss, Purchased with Credit Deterioration					29
Financing Receivable, Allowance for Credit Loss, Writeoff			(1,182)		(1,625)	(1,598)
Financing Receivable, Allowance for Credit Loss, Recovery			638		1,035	787
Financing Receivable, Credit Loss, Expense (Reversal)			448		423	$ 785
Financing Receivable, Nonaccrual, No Allowance			0		0
Finance Receivable, Nonaccrual, with allowance			504		156
Financing Receivable, Nonaccrual	[1]		$ 504		$ 156

[1] Included in these amounts are $2.0 million and $22.2 million of nonaccruing TDRs at December 31, 2021 and December 31, 2020, respectively.