Investment Securities - Amortized Cost, Debt Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,641,277	$ 1,558,787
Gross Unrealized Gains	49,985	13,596
Gross Unrealized Losses	(1,874)	(5,582)
Fair Value	1,689,388	1,566,801
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	50,940	50,952
Gross Unrealized Gains	1,681	12
Gross Unrealized Losses	0	(21)
Fair Value	52,621	50,943
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	26,778	28,982
Gross Unrealized Gains	1,174	280
Gross Unrealized Losses	(28)	(19)
Fair Value	27,924	29,243
Obligations of states and state agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	65,620	78,116
Gross Unrealized Gains	1,477	540
Gross Unrealized Losses	(54)	(83)
Fair Value	67,043	78,573
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	79,100	90,662
Gross Unrealized Gains	1,363	902
Gross Unrealized Losses	(1)	(86)
Fair Value	80,462	91,478
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	144,720	168,778
Gross Unrealized Gains	2,840	1,442
Gross Unrealized Losses	(55)	(169)
Fair Value	147,505	170,051
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,340,614	1,248,814
Gross Unrealized Gains	43,039	11,234
Gross Unrealized Losses	(927)	(5,262)
Fair Value	1,382,726	1,254,786
Corporate and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	78,225	61,261
Gross Unrealized Gains	1,251	628
Gross Unrealized Losses	(864)	(111)
Fair Value	$ 78,612	$ 61,778