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Subsequent Events	3 Months Ended
Subsequent Events	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events The Company has evaluated events subsequent to December 31, 2017 and through the financial statement issuance date of June 15, 2018. On April 19, 2018, the Company completed its minority stock offering, after receiving all regulatory approvals with trading commencing on the Nasdaq Global Select Market on April 20, 2018. In connection with the closing, the Company issued 62,580,155 shares of its common stock to the MHC, the Company's mutual holding company, 3,476,675 shares to the Columbia Bank Foundation, the Bank's charitable foundation, and 49,832,345 shares to depositors of the Bank who subscribed for and were allocated shares in the minority stock offering, as well as the Columbia Bank Employee Stock Ownership Plan.