Acquisition Fair value of assets acquired and liabilities assumed (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Apr. 01, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill	$ 504,562		$ 256,105
Blue Hills Bancorp, Inc. [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash		$ 56,331
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Investments		196,937
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Loans Net		2,073,714
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment		24,253
Goodwill		248,457
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles		19,870
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Assets		147,836
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Financial Assets		2,767,398
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits		1,930,436
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Borrowings		124,817
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Liabilities		50,857
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Financial Liabilities		2,106,110
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net		$ 661,288