Investment Securities - Credit Quality Indicator (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	$ 3,759,968	$ 3,761,094
State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	1,859,956	1,861,102
Other securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	255,157	$ 261,199
Aaa/AAA | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	478,643
Aaa/AAA | Other securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	0
Aa/AA | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	1,160,675
Aa/AA | Other securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	0
A | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	208,553
A | Other securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	101,965
Baa/BBB | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	4,371
Baa/BBB | Other securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	153,192
Not Rated | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	7,714
Not Rated | Other securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	0
Not Guaranteed or Pre-Refunded | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	871,985
Not Guaranteed or Pre-Refunded | Aaa/AAA | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	178,809
Not Guaranteed or Pre-Refunded | Aa/AA | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	638,308
Not Guaranteed or Pre-Refunded | A | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	47,154
Not Guaranteed or Pre-Refunded | Baa/BBB | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	0
Not Guaranteed or Pre-Refunded | Not Rated | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	7,714
Other Credit Enhancement or Insurance | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	987,971
Other Credit Enhancement or Insurance | Aaa/AAA | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	299,834
Other Credit Enhancement or Insurance | Aa/AA | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	522,367
Other Credit Enhancement or Insurance | A | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	161,399
Other Credit Enhancement or Insurance | Baa/BBB | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	4,371
Other Credit Enhancement or Insurance | Not Rated | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	0
Pre-Refunded | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	0
Pre-Refunded | Aaa/AAA | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	0
Pre-Refunded | Aa/AA | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	0
Pre-Refunded | A | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	0
Pre-Refunded | Baa/BBB | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	0
Pre-Refunded | Not Rated | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	$ 0