Investment Securities - Credit Quality Indicator (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	$ 3,688,664	$ 3,729,502
State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	1,857,600	1,858,680
Other securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	256,835	$ 256,007
Aaa/AAA | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	479,458
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	1,158,695
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	198,582
A | Other securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	102,815
Baa/BBB | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	4,381
Baa/BBB | Other securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	154,020
Not Rated | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	16,484
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	866,242
Not Guaranteed or Pre-Refunded | Aaa/AAA | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	179,308
Not Guaranteed or Pre-Refunded | Aa/AA | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	633,415
Not Guaranteed or Pre-Refunded | A | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	37,035
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	16,484
Other Credit Enhancement or Insurance | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	991,358
Other Credit Enhancement or Insurance | Aaa/AAA | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	300,150
Other Credit Enhancement or Insurance | Aa/AA | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	525,280
Other Credit Enhancement or Insurance | A | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	161,547
Other Credit Enhancement or Insurance | Baa/BBB | State and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	4,381
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