Loans Receivable and Allowance for Loan Losses - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Allowance for loan losses:
Total	$ 74,015	$ 71,200	$ 61,709	$ 62,403	$ 62,771	$ 62,342
Total loans:
Total	6,632,347		6,176,329
Real estate loans | One-to-four family
Allowance for loan losses:
Total	16,633	16,798	13,780	15,610	17,375	15,232
Total loans:
Total	2,164,482		2,077,508
Real estate loans | Multifamily and commercial
Allowance for loan losses:
Total	27,330	26,085	22,980	22,679	20,986	23,251
Total loans:
Total	2,878,965		2,924,851
Real estate loans | Construction
Allowance for loan losses:
Total	10,217	9,399	7,435	8,806	9,033	7,217
Total loans:
Total	326,343		298,942
Commercial business loans
Allowance for loan losses:
Total	18,314	17,191	15,836	12,718	12,225	14,176
Total loans:
Total	901,144		484,941
Consumer loans | Home equity loans and advances
Allowance for loan losses:
Total	1,514	1,718	1,669	2,583	3,146	2,458
Total loans:
Total	359,813		388,127
Consumer loans | Other consumer loans
Allowance for loan losses:
Total	7	$ 9	9	$ 7	$ 6	$ 8
Total loans:
Total	1,600		1,960
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	2,129		1,621
Allowance for loan losses, Collectively evaluated for impairment	71,886		60,088
Total loans:
Total loans, individually evaluated for impairment	31,071		18,811
Total loans, collectively evaluated for impairment	6,594,395		6,150,497
Total	6,625,466		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	434		484
Allowance for loan losses, Collectively evaluated for impairment	16,199		13,296
Total loans:
Total loans, individually evaluated for impairment	8,810		8,891
Total loans, collectively evaluated for impairment	2,155,375		2,068,188
Total	2,164,185		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	584		2
Allowance for loan losses, Collectively evaluated for impairment	26,746		22,978
Total loans:
Total loans, individually evaluated for impairment	14,960		2,599
Total loans, collectively evaluated for impairment	2,859,059		2,917,386
Total	2,874,019		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	10,217		7,435
Total loans:
Total loans, individually evaluated for impairment	0		0
Total loans, collectively evaluated for impairment	326,343		298,942
Total	326,343		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	1,100		1,121
Allowance for loan losses, Collectively evaluated for impairment	17,214		14,715
Total loans:
Total loans, individually evaluated for impairment	5,293		5,178
Total loans, collectively evaluated for impairment	894,213		478,037
Total	899,506		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	11		14
Allowance for loan losses, Collectively evaluated for impairment	1,503		1,655
Total loans:
Total loans, individually evaluated for impairment	2,008		2,143
Total loans, collectively evaluated for impairment	357,805		385,984
Total	359,813		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	1,600		1,960
Total	1,600		1,960
Financial Asset Acquired with Credit Deterioration
Allowance for loan losses:
Total	0		0
Total loans:
Total	6,881		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	297		429
Financial Asset Acquired with Credit Deterioration | Real estate loans | Multifamily and commercial
Allowance for loan losses:
Total	0		0
Total loans:
Total	4,946		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	1,638		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