Loans Receivable and Allowance for Loan Losses - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Allowance for loan losses:
Total	$ 76,133	$ 74,015	$ 61,709	$ 62,629	$ 62,403	$ 62,342
Total loans:
Total	6,458,807		6,176,329
Real estate loans | One-to-four family
Allowance for loan losses:
Total	15,775	16,633	13,780	15,733	15,610	15,232
Total loans:
Total	2,053,584		2,077,508
Real estate loans | Multifamily and commercial
Allowance for loan losses:
Total	30,989	27,330	22,980	22,969	22,679	23,251
Total loans:
Total	2,868,641		2,924,851
Real estate loans | Construction
Allowance for loan losses:
Total	10,534	10,217	7,435	7,760	8,806	7,217
Total loans:
Total	307,659		298,942
Commercial business loans
Allowance for loan losses:
Total	17,502	18,314	15,836	14,028	12,718	14,176
Total loans:
Total	886,423		484,941
Consumer loans | Home equity loans and advances
Allowance for loan losses:
Total	1,326	1,514	1,669	2,131	2,583	2,458
Total loans:
Total	340,962		388,127
Consumer loans | Other consumer loans
Allowance for loan losses:
Total	7	$ 7	9	$ 8	$ 7	$ 8
Total loans:
Total	1,538		1,960
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	1,839		1,621
Allowance for loan losses, Collectively evaluated for impairment	74,294		60,088
Total loans:
Total loans, individually evaluated for impairment	45,846		18,811
Total loans, collectively evaluated for impairment	6,406,255		6,150,497
Total	6,452,101		6,169,308
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | One-to-four family
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	406		484
Allowance for loan losses, Collectively evaluated for impairment	15,369		13,296
Total loans:
Total loans, individually evaluated for impairment	7,230		8,891
Total loans, collectively evaluated for impairment	2,046,063		2,068,188
Total	2,053,293		2,077,079
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Multifamily and commercial
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	550		2
Allowance for loan losses, Collectively evaluated for impairment	30,439		22,978
Total loans:
Total loans, individually evaluated for impairment	32,090		2,599
Total loans, collectively evaluated for impairment	2,831,628		2,917,386
Total	2,863,718		2,919,985
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Construction
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	0		0
Allowance for loan losses, Collectively evaluated for impairment	10,534		7,435
Total loans:
Total loans, individually evaluated for impairment	0		0
Total loans, collectively evaluated for impairment	307,659		298,942
Total	307,659		298,942
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Commercial business loans
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	871		1,121
Allowance for loan losses, Collectively evaluated for impairment	16,631		14,715
Total loans:
Total loans, individually evaluated for impairment	4,664		5,178
Total loans, collectively evaluated for impairment	880,267		478,037
Total	884,931		483,215
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Home equity loans and advances
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	12		14
Allowance for loan losses, Collectively evaluated for impairment	1,314		1,655
Total loans:
Total loans, individually evaluated for impairment	1,862		2,143
Total loans, collectively evaluated for impairment	339,100		385,984
Total	340,962		388,127
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	0		0
Allowance for loan losses, Collectively evaluated for impairment	7		9
Total loans:
Total loans, individually evaluated for impairment	0		0
Total loans, collectively evaluated for impairment	1,538		1,960
Total	1,538		1,960
Financial Asset Acquired with Credit Deterioration
Allowance for loan losses:
Total	0		0
Total loans:
Total	6,706		7,021
Financial Asset Acquired with Credit Deterioration | Real estate loans | One-to-four family
Allowance for loan losses:
Total	0		0
Total loans:
Total	291		429
Financial Asset Acquired with Credit Deterioration | Real estate loans | Multifamily and commercial
Allowance for loan losses:
Total	0		0
Total loans:
Total	4,923		4,866
Financial Asset Acquired with Credit Deterioration | Real estate loans | Construction
Allowance for loan losses:
Total	0		0
Total loans:
Total	0		0
Financial Asset Acquired with Credit Deterioration | Commercial business loans
Allowance for loan losses:
Total	0		0
Total loans:
Total	1,492		1,726
Financial Asset Acquired with Credit Deterioration | Consumer loans | Home equity loans and advances
Allowance for loan losses:
Total	0		0
Total loans:
Total	0		0
Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans
Allowance for loan losses:
Total	0		0
Total loans:
Total	$ 0		$ 0