Employee Benefit Plans - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended	12 Months Ended
	Apr. 30, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, years of employment			5 years
Bank-owned life insurance			$ 211,415,000	$ 184,488,000
Bank-owned life insurance income		$ 1,089,000	5,846,000	5,208,000	$ 4,936,000
Defined benefit plan, accumulated benefit obligation			14,100,000	10,500,000
Defined contribution plan, cost		159,000	1,100,000	1,300,000	395,000
Proceeds from repayments of loans by employee stock ownership plans		0	0	45,428,000	0
Employee stock ownership plan, compensation expense		0	3,566,000	2,594,000	0
RIM
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, funded (unfunded) status of plan			(13,696,000)	(11,285,000)
Defined contribution plan, cost		1,000	11,000	9,000	14,000
Employee stock ownership plan, compensation expense		0	267,000	165,000	0
Post-retirement
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, funded (unfunded) status of plan			(24,603,000)	(20,964,000)
Pension
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, funded (unfunded) status of plan			92,521,000	63,769,000
Defined contribution plan, cost		$ 289,000	$ 1,400,000	1,300,000	1,200,000
Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of match			4.50%
Maximum | Pension
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of match			4.50%
Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of match			3.00%
Minimum | Pension
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of match			3.00%
Columbia Bank Employee Stock Ownership Plan
Defined Benefit Plan Disclosure [Line Items]
Proceeds from repayments of loans by employee stock ownership plans	$ 45,400,000
Loan term	20 years		20 years
Shares contributed to ESOP (in shares)	4,542,855
Fixed interest rate	4.75%
Employee stock ownership plan, compensation expense			$ 3,600,000	$ 2,600,000	$ 0