Parents Company Financial Statements - Statements of Cash Flows (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating activities
Net income		$ 154,665	$ 22,738	$ 135,098
Adjustments to reconcile net income to net cash provided by operating activities
ESOP expense		9,408	2,351	0
Net cash provided by operating activities		174,490	69,851	196,223
Investing activities
Acquisitions, net of cash and cash equivalents acquired		(13,439)	(1,363)	0
Net cash used in investing activities		(1,985,744)	(2,390,183)	(145,274)
Financing activities
Proceeds from issuance of common shares		0	1,792,878	0
Payments for deferred offering costs		0	(28,552)	(346)
Payment of subordinated debentures assumed in business combination	[1]	(36,277)	0	0
Payments for shares repurchased under share repurchase plan		(23,224)	0	0
Net cash provided by financing activities		988,976	4,011,800	51,945
Net (decrease) increase in cash, cash equivalents, and restricted cash		(822,278)	1,691,468	102,894
Cash, cash equivalents, and restricted cash at beginning of period		2,054,070	362,602	259,708
Cash, cash equivalents, and restricted cash at end of period		1,231,792	2,054,070	362,602
Parent
Operating activities
Net income		154,665	22,738	135,098
Adjustments to reconcile net income to net cash provided by operating activities
Equity in undistributed income of subsidiaries		(150,936)	(101,728)	(135,432)
Issuance of common shares donated to the Eastern Bank Foundation		0	91,287	0
ESOP expense		9,408	2,351	0
Deferred income taxes, net		(7,157)	(10,817)	0
Other, net		(388)	(350)	25
Net cash provided by operating activities		5,592	3,481	(309)
Investing activities
Investment in Eastern Bank		0	(882,096)	0
Acquisitions, net of cash and cash equivalents acquired		(640,890)	0	0
Proceeds from Equity Method Investment, Distribution, Return of Capital		140,000	0	0
Net cash used in investing activities		(500,890)	(882,096)	0
Financing activities
Proceeds from issuance of common shares		0	1,792,878	0
Purchase of shares by ESOP		0	(149,407)	0
Payments for deferred offering costs		0	(28,552)	(346)
Payment of subordinated debentures assumed in business combination		(36,277)	0	0
Payments for shares repurchased under share repurchase plan		(23,224)	0	0
Dividends declared and paid to common shareholders		(51,564)	0	0
Net cash provided by financing activities		(111,065)	1,614,919	(346)
Net (decrease) increase in cash, cash equivalents, and restricted cash		(606,363)	736,304	(655)
Cash, cash equivalents, and restricted cash at beginning of period		741,034	4,730	5,385
Cash, cash equivalents, and restricted cash at end of period		$ 134,671	$ 741,034	$ 4,730

[1] The Company deposited funds into escrow prior to the Century acquisition date to pay the balance of subordinated debentures assumed in the Century acquisition which was considered to be a defeasance of the debt. Accordingly, Century recorded a payable to the Company in the amount of the escrow deposit and the Company recorded a receivable from Century in the same amount. The payable was reclassified to other assets upon acquisition and is reflected as such balance in the summary of net assets acquired included in Note 3 to the consolidated financial statements. Subsequent to the closing of the acquisition and prior to December 31, 2021, the amounts placed in escrow were disbursed to the holders of the subordinated debentures resulting in a full pay-off of the outstanding balance of the debt.