Acquisition (Details) - USD ($)		3 Months Ended		9 Months Ended
	Apr. 01, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Business Acquisition [Line Items]
Business Combination, Acquired Receivables, Description	$ 23,200,000
Business Combination, Acquisition Related Costs		$ 705,000	$ 2,688,000	$ 26,433,000	$ 3,122,000
Blue Hills Bancorp, Inc. [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares of Company Stock Issued for Each Share of Acquiree Stock	0.2308
Business Combination, Increase in Acquirer Outstanding Shares	6,166,010
Payments to Acquire Businesses, Gross	$ 161,600,000
Business Combination, Acquisition Related Costs				26,000,000.0
Business Acquisition, Equity Interest Issued or Issuable Shareholders Option To Receive, Cash Per Share	$ 5.25
Business Combination, Cost of Acquired Entity, Purchase Price	$ 661,300,000
Combination of INDB and Blue Hills Bancorp, Inc. [Member]
Business Acquisition [Line Items]
Business Combination, Acquisition Related Costs		$ 505,000		$ 57,300,000