Summary of Significant Accounting Policies (Details) - USD ($)	1 Months Ended	12 Months Ended
	Apr. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Nov. 30, 2019	Jan. 01, 2019	Aug. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Junior subordinated debenture owed to unconsolidated subsidiary trust						$ 51,500,000
Assets		$ 8,188,694,000	$ 6,691,618,000
Cash reserves		8,800,000	5,900,000
Loan threshold for individual evaluation for impairment		500,000
Equity securities, at fair value		$ 2,855,000	1,890,000
Retained Earnings
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification from AOCI to retained earnings due to TCJA			11,700,000
Buildings
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Property, plant and equipment, useful life		40 years
Minimum
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Defined contribution plan, employer matching contribution, percent of match		3.00%
Minimum | Land and building improvements
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Property, plant and equipment, useful life		10 years
Minimum | Furniture and equipment
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Property, plant and equipment, useful life		3 years
Maximum
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Defined contribution plan, employer matching contribution, percent of match		4.50%
Maximum | Land and building improvements
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Property, plant and equipment, useful life		20 years
Maximum | Furniture and equipment
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Property, plant and equipment, useful life		10 years
Accounting Standards Update 2016-01
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Equity securities, at fair value			$ 1,900,000
Reclassification of tax effect resulting from adoption					$ 0
Accounting Standards Update 2016-01 | Retained Earnings
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification of tax effect resulting from adoption					$ 548,000
Columbia Financial Capital Trust I
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Ownership percentage by parent		100.00%
Stewardship Statutory Trust I
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Ownership percentage by parent		100.00%
Columbia Bank Employee Stock Ownership Plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Loan term	20 years	20 years
Stewardship Property
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Assets held-for-sale, long-lived, fair value disclosure		$ 1,800,000
Variable Interest Entity, Not Primary Beneficiary
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Assets				$ 7,000,000.0