Loans and Allowance for Loan Losses, Based on Impairment Review Method (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Ending allowance balance attributable to loans [Abstract]
Individually evaluated for impairment	$ 0		$ 0
Collectively evaluated for impairment	47,350		49,595
Total ending allowance balance	47,350	$ 50,155	49,595	$ 49,123	$ 48,144	$ 44,317
Loans [Abstract]
Individually evaluated for impairment	19,813		21,581
Collectively evaluated for impairment	4,376,916		4,222,889
Total loans	4,396,729		4,244,470
Commercial [Member]
Ending allowance balance attributable to loans [Abstract]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	3,253		4,140
Total ending allowance balance	3,253	4,106	4,140	4,267	4,366	3,999
Loans [Abstract]
Individually evaluated for impairment	521		1,028
Collectively evaluated for impairment	204,158		211,464
Total loans	204,679		212,492
1 to 4 Family Residential Real Estate [Member]
Ending allowance balance attributable to loans [Abstract]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	43,653		44,950
Total ending allowance balance	43,653	45,617	44,950	44,323	43,274	39,748
Loans [Abstract]
Individually evaluated for impairment	19,292		20,553
Collectively evaluated for impairment	4,163,307		4,001,808
Total loans	4,182,599		4,022,361
Installment [Member]
Ending allowance balance attributable to loans [Abstract]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	444		505
Total ending allowance balance	444	$ 432	505	$ 533	$ 504	$ 570
Loans [Abstract]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	9,451		9,617
Total loans	$ 9,451		$ 9,617