Retirement Benefits - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Requisite service period for employees to be eligible for 401(k) plan	3 months
Percent of employer match on employee contributions	25.00%
Percent of base compensation contributed by eligible employees	6.00%
Contribution plan participation period (years)	3 years
Contribution plan subsequent participation period (years)	3 years
Requisite service period for members to become fully vested	5 years
Company contribution amount	$ 426,000	$ 394,000	$ 387,000
ESOP compensation expense	1,900,000	2,000,000.0	2,200,000
Estimated loss that will be amortized from accumulated other comprehensive income (loss) into net periodic cost	835
Prior service cost that will be amortized from accumulated other comprehensive income (loss) into net periodic cost	(19,000)
Transition obligation that will be amortized from accumulated other comprehensive income (loss) into net periodic cost	$ 0
Medical cost trend rate	8.75%
Medical cost trend rate reduction percent per year	0.50%
Medical cost trend rate ultimate rate	4.75%
Benefits paid	$ 103,000	120,000	112,000
Benefits expected to be paid under the postretirement health benefits plan in 2020	108,000
Benefits expected to be paid under the postretirement health benefits plan in 2021	108,000
Benefits expected to be paid under the postretirement health benefits plan in 2022	107,000
Benefits expected to be paid under the postretirement health benefits plan in 2023	88,000
Benefits expected to be paid under the postretirement health benefits plan in 2024	85,000
Benefits expected to be paid under the postretirement health benefits plan in 2025 through 2029	359,000
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	14,700,000	12,700,000
Net postretirement benefit cost included in compensation and employee benefits	$ 2,000,000.0	(879,000)	1,100,000
Minimum
Defined Benefit Plan Disclosure [Line Items]
Percent of compensation employees can invest in 401(k) plan	2.00%
Matching contribution percent	25.00%
Maximum
Defined Benefit Plan Disclosure [Line Items]
Percent of compensation employees can invest in 401(k) plan	15.00%
Matching contribution percent	50.00%
Northfield Bank ESOP
Defined Benefit Plan Disclosure [Line Items]
Employee stock ownership plan, shares authorized and purchased	2,463,884
Initial public offering price (dollars per share)	$ 7.13
Outstanding loan balance	$ 10,100,000	$ 10,800,000
Shares released and allocated to participants (in shares)	100,841	101,514
Second ESOP
Defined Benefit Plan Disclosure [Line Items]
Employee stock ownership plan, shares authorized and purchased	1,422,357
Initial public offering price (dollars per share)	$ 10.00
Outstanding loan balance	$ 11,800,000	$ 12,200,000
Shares released and allocated to participants (in shares)	53,320	52,523
Supplemental ESOP
Defined Benefit Plan Disclosure [Line Items]
Company contribution to SESOP plan	$ 41,000	$ 76,000	$ 84,000