Employee Benefit Plans - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, years of employment				5 years
Bank-owned life insurance		$ 184,488,000	$ 150,521,000	$ 184,488,000
Bank-owned life insurance income			1,089,000	5,208,000	$ 4,936,000	$ 4,370,000
Defined benefit plan, accumulated benefit obligation		10,500,000	5,900,000	10,500,000
Defined contribution plan, cost			159,000	1,342,000	395,000	356,000
Proceeds from repayments of loans by employee stock ownership plans			0	45,428,000	0	0
Employee stock ownership plan, compensation expense			0	2,594,000	0	0
RIM
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, funded (unfunded) status of plan		(11,285,000)	(12,243,000)	(11,285,000)
Defined contribution plan, cost			1,000	86,000	14,000	47,000
Employee stock ownership plan, compensation expense				165,000
Deferred Compensation Arrangement with Individual, Compensation Expense		0			0	0
Pension
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, funded (unfunded) status of plan		$ 63,769,000	60,234,000	63,769,000
Defined contribution plan, cost			289,000	$ 1,342,000	1,245,000	1,142,000
Maximum | Pension
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of match				4.50%
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
Shares contributed to ESOP	4,542,855
Fixed interest rate	4.75%
Employee stock ownership plan, compensation expense			$ 0		$ 0	$ 0