Securities - Gross Unrealized Losses and Fair Value of Company's Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	$ 1,430,764	$ 79,860
Unrealized losses less than 12 months	(22,640)	(294)
Fair value more than 12 months	8,047	1,245
Unrealized losses more than 12 months	(246)	(29)
Total fair value	1,438,811	81,105
Total unrealized losses	(22,886)	(323)
U.S. Treasury and federal agencies
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	108,551	17,215
Unrealized losses less than 12 months	(688)	(35)
Fair value more than 12 months	2,731	0
Unrealized losses more than 12 months	(19)	0
Total fair value	111,282	17,215
Total unrealized losses	(707)	(35)
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	680,184	56,287
Unrealized losses less than 12 months	(13,892)	(242)
Fair value more than 12 months	5,316	1,245
Unrealized losses more than 12 months	(227)	(29)
Total fair value	685,500	57,532
Total unrealized losses	(14,119)	(271)
Federal agency collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	57,191	6,358
Unrealized losses less than 12 months	(797)	(17)
Fair value more than 12 months	0	0
Unrealized losses more than 12 months	0	0
Total fair value	57,191	6,358
Total unrealized losses	(797)	$ (17)
Federal agency mortgage-backed pools
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	363,453
Unrealized losses less than 12 months	(4,610)
Fair value more than 12 months	0
Unrealized losses more than 12 months	0
Total fair value	363,453
Total unrealized losses	(4,610)
Private labeled mortgage-backed pools
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	77,208
Unrealized losses less than 12 months	(1,038)
Fair value more than 12 months	0
Unrealized losses more than 12 months	0
Total fair value	77,208
Total unrealized losses	(1,038)
Corporate notes
Schedule of Available-for-sale Securities [Line Items]
Fair Value less than 12 months	144,177
Unrealized losses less than 12 months	(1,615)
Fair value more than 12 months	0
Unrealized losses more than 12 months	0
Total fair value	144,177
Total unrealized losses	$ (1,615)