Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities Available for Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,293,506	$ 1,558,787
Gross Unrealized Gains	46,886	13,596
Gross Unrealized Losses	(919)	(5,582)
Fair Value	1,339,473	1,566,801
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	50,031	50,952
Gross Unrealized Gains	1,362	12
Gross Unrealized Losses	0	(21)
Fair Value	51,393	50,943
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	25,067	28,982
Gross Unrealized Gains	1,103	280
Gross Unrealized Losses	(13)	(19)
Fair Value	26,157	29,243
Obligations of states and state agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	40,861	78,116
Gross Unrealized Gains	970	540
Gross Unrealized Losses	(32)	(83)
Fair Value	41,799	78,573
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	37,489	90,662
Gross Unrealized Gains	731	902
Gross Unrealized Losses	(69)	(86)
Fair Value	38,151	91,478
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	78,350	168,778
Gross Unrealized Gains	1,701	1,442
Gross Unrealized Losses	(101)	(169)
Fair Value	79,950	170,051
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,050,369	1,248,814
Gross Unrealized Gains	40,426	11,234
Gross Unrealized Losses	(773)	(5,262)
Fair Value	1,090,022	1,254,786
Corporate and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	89,689	61,261
Gross Unrealized Gains	2,294	628
Gross Unrealized Losses	(32)	(111)
Fair Value	$ 91,951	$ 61,778