Fair Value Measurement - Recurring and Nonrecurring Basis (Details) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		$ 2,753,104	$ 2,587,305
Derivative assets	[1]	14,345	18,537
Derivative liabilities	[1]	1,520	902
US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		43,547
State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		1,967,477	1,580,594
Corporate bonds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		119,699	78,255
Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[2]	514,078	810,010
Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[2]	108,303	118,446
Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments		5,976	6,094
Total asset fair value measurements		2,775,430	2,611,936
Total liability fair value measurements		24,905	40,172
Fair Value, Recurring | US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		43,547
Fair Value, Recurring | State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		1,967,477	1,580,594
Fair Value, Recurring | Corporate bonds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		119,699	78,255
Fair Value, Recurring | Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[3]	514,078	810,010
Fair Value, Recurring | Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[3]	108,303	118,446
Fair Value, Recurring | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		16,350	18,537
Derivative liabilities		24,905	40,172
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments		5,976	6,094
Total asset fair value measurements		49,523	6,094
Total liability fair value measurements		0	0
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		43,547
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0	0
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0	0
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[3]	0	0
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[3]	0	0
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		0	0
Derivative liabilities		0	0
Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments		0	0
Total asset fair value measurements		2,725,907	2,605,842
Total liability fair value measurements		24,905	40,172
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		1,967,477	1,580,594
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Corporate bonds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		119,699	78,255
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[3]	514,078	810,010
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[3]	108,303	118,446
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		16,350	18,537
Derivative liabilities		24,905	40,172
Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments		0	0
Total asset fair value measurements		0	0
Total liability fair value measurements		0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Corporate bonds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[3]	0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[3]	0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		0	0
Derivative liabilities		0	0
Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed assets		40	120
Total asset fair value measurements		8,619	10,773
Fair Value, Nonrecurring | Collateral-dependent loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral-dependent loans	[4]	8,579	10,653
Fair Value, Nonrecurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed assets		0	0
Total asset fair value measurements		0	0
Fair Value, Nonrecurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Collateral-dependent loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral-dependent loans	[4]	0	0
Fair Value, Nonrecurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed assets		0	0
Total asset fair value measurements		0	0
Fair Value, Nonrecurring | Significant Other Observable Inputs (Level 2) | Collateral-dependent loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral-dependent loans	[4]	0	0
Fair Value, Nonrecurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed assets		40	120
Total asset fair value measurements		8,619	10,773
Fair Value, Nonrecurring | Significant Unobservable Inputs (Level 3) | Collateral-dependent loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral-dependent loans	[4]	$ 8,579	$ 10,653

[1] Net derivative assets are included in other assets and net derivative liabilities are included in other liabilities on the consolidated balance sheets. Included in the fair value of net derivative assets and net derivative liabilities are credit valuation adjustments reflecting counterparty credit risk and our credit risk. We had no credit exposure related to interest rate swaps with financial institutions and $14.3 million related to interest rate swaps with customers at September 30, 2021. We had no credit exposure related to interest rate swaps with financial institutions and $18.5 million related to interest rate swaps with customers at December 31, 2020. The credit risk associated with customer transactions is partially mitigated as these are generally secured by the non-cash collateral securing the underlying transaction being hedged.

[2] All MBS are issued and/or guaranteed by U.S. government agencies or U.S. GSEs.

[3] All MBS are issued and/or guaranteed by U.S. government agencies or U.S. GSEs.

[4] Consists of individually evaluated loans. Loans for which the fair value of the collateral and commercial real estate fair value of the properties is less than cost basis are presented net of allowance. Losses on these loans represent charge-offs which are netted against the allowance for loan losses.