Investment Securities - Amortized Cost of Investment Securities Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,071,183	$ 1,227,668
Aaa
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	151,669
Aa1
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	146,992
Aa2
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	161,730
Aa3
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	124,369
A1
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	88,277
A2
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	23,103
A3
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,066
Baa2
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	266
Non-rated
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,373,711
State and municipal
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	996,247	$ 619,927
State and municipal | Aaa
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	90,991
State and municipal | Aa1
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	146,992
State and municipal | Aa2
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	161,730
State and municipal | Aa3
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	124,369
State and municipal | A1
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	88,277
State and municipal | A2
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	23,103
State and municipal | A3
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,066
State and municipal | Baa2
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	266
State and municipal | Non-rated
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	359,453
Other
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,074,936
Other | Aaa
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	60,678
Other | Aa1
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0
Other | Aa2
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0
Other | Aa3
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0
Other | A1
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0
Other | A2
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0
Other | A3
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0
Other | Baa2
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0
Other | Non-rated
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,014,258