Investment Securities - Amortized Cost of Debt Securities Held-to-Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Amortized cost, held to maturity	$ 337,165	$ 375,606
AAA/AA/A
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Amortized cost, held to maturity	336,472	374,921
BBB/BB/B
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Amortized cost, held to maturity	693	685
Obligations of U.S. government agencies | AAA/AA/A
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Amortized cost, held to maturity	324,976	361,785
Obligations of U.S. government agencies | BBB/BB/B
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Amortized cost, held to maturity	0	0
Obligations of states and political subdivisions | AAA/AA/A
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Amortized cost, held to maturity	11,496	13,136
Obligations of states and political subdivisions | BBB/BB/B
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Amortized cost, held to maturity	$ 693	$ 685