Parent Company Only Financial Information - Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Cash and cash equivalents at subsidiary bank	$ 43,305	$ 154,353
Investment in subsidiary bank	318,926	285,155
Other assets	3,595	2,509
Total Assets	3,042,464	2,963,360
LIABILITIES AND STOCKHOLDERS' EQUITY
Liabilities	2,658,712	2,608,336
Redeemable common stock held by ESOP	48,870	43,308
Stockholders' equity:
Capital stock	0	0
Retained earnings	371,848	341,558
Accumulated other comprehensive loss	(3,250)	(2,446)
Treasury stock at cost	(36,968)	(33,018)
Total Stockholders' Equity	383,752	355,024
Total Stockholders' Equity Less Maximum Cash Obligations Related To ESOP Shares	334,882	311,716	$ 289,270	$ 272,175
Total Liabilities & Stockholders' Equity	3,042,464	2,963,360
Parent Company [Member]
ASSETS
Cash and cash equivalents at subsidiary bank	3,742	4,015	$ 543	$ 843
Investment in subsidiary bank	385,193	355,456
Other assets	1,231	1,393
Total Assets	390,166	360,864
LIABILITIES AND STOCKHOLDERS' EQUITY
Liabilities	6,414	5,840
Redeemable common stock held by ESOP	48,870	43,308
Stockholders' equity:
Capital stock	52,122	48,930
Retained earnings	371,848	341,558
Accumulated other comprehensive loss	(3,250)	(2,446)
Treasury stock at cost	(36,968)	(33,018)
Total Stockholders' Equity	383,752	355,024
Less maximum cash obligation related to ESOP shares	48,870	43,308
Total Stockholders' Equity Less Maximum Cash Obligations Related To ESOP Shares	334,882	311,716
Total Liabilities & Stockholders' Equity	$ 390,166	$ 360,864