Investment Securities - Amortized Cost, Debt Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,479,966	$ 1,558,787
Gross Unrealized Gains	47,999	13,596
Gross Unrealized Losses	(1,401)	(5,582)
Fair Value	1,526,564	1,566,801
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	50,035	50,952
Gross Unrealized Gains	1,543	12
Gross Unrealized Losses	0	(21)
Fair Value	51,578	50,943
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	25,302	28,982
Gross Unrealized Gains	1,164	280
Gross Unrealized Losses	(18)	(19)
Fair Value	26,448	29,243
Obligations of states and state agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	58,807	78,116
Gross Unrealized Gains	1,151	540
Gross Unrealized Losses	(30)	(83)
Fair Value	59,928	78,573
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	73,063	90,662
Gross Unrealized Gains	1,276	902
Gross Unrealized Losses	(74)	(86)
Fair Value	74,265	91,478
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	131,870	168,778
Gross Unrealized Gains	2,427	1,442
Gross Unrealized Losses	(104)	(169)
Fair Value	134,193	170,051
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,193,052	1,248,814
Gross Unrealized Gains	41,326	11,234
Gross Unrealized Losses	(942)	(5,262)
Fair Value	1,233,436	1,254,786
Corporate and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	79,707	61,261
Gross Unrealized Gains	1,539	628
Gross Unrealized Losses	(337)	(111)
Fair Value	$ 80,909	$ 61,778