Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Debt Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,084,582	$ 1,293,506
Gross Unrealized Gains	33,055	46,886
Gross Unrealized Losses	(1,416)	(919)
Fair Value	1,116,221	1,339,473
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	50,027	50,031
Gross Unrealized Gains	1,172	1,362
Gross Unrealized Losses	0	0
Fair Value	51,199	51,393
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22,899	25,067
Gross Unrealized Gains	1,123	1,103
Gross Unrealized Losses	(12)	(13)
Fair Value	24,010	26,157
Obligations of states and state agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	36,075	40,861
Gross Unrealized Gains	793	970
Gross Unrealized Losses	0	(32)
Fair Value	36,868	41,799
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	35,223	37,489
Gross Unrealized Gains	637	731
Gross Unrealized Losses	(6)	(69)
Fair Value	35,854	38,151
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	71,298	78,350
Gross Unrealized Gains	1,430	1,701
Gross Unrealized Losses	(6)	(101)
Fair Value	72,722	79,950
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	852,398	1,050,369
Gross Unrealized Gains	27,254	40,426
Gross Unrealized Losses	(1,316)	(773)
Fair Value	878,336	1,090,022
Corporate and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	87,960	89,689
Gross Unrealized Gains	2,076	2,294
Gross Unrealized Losses	(82)	(32)
Fair Value	$ 89,954	$ 91,951