Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Held-to-Maturity
Amortized Cost	$ 47,475		$ 40,927
Allowance for credit losses on HTM	(373)	$ (307)	0
Net Carrying Amount	47,102		40,927
Gross Unrealized Gains	1,964		987
Gross Unrealized (Losses)	(2)		(59)
Estimated Fair Value	49,064		41,855
Available-for-sale
Amortized Cost	2,556,808		3,263,151
Allowance for credit losses on AFS	(1,208)	(609)	0
Gross Unrealized Gains	56,737		31,740
Gross Unrealized (Losses)	(5,049)		(6,548)
Estimated Fair Value	2,607,288		3,288,343
U.S. Treasury
Available-for-sale
Amortized Cost			449,729
Allowance for credit losses on AFS			0
Gross Unrealized Gains			112
Gross Unrealized (Losses)			(112)
Estimated Fair Value			449,729
U.S. Government agencies
Available-for-sale
Amortized Cost	472,078		194,207
Allowance for credit losses on AFS	0		0
Gross Unrealized Gains	1,257		1,313
Gross Unrealized (Losses)	(1,362)		(1,271)
Estimated Fair Value	471,973		194,249
Mortgage-backed securities
Held-to-Maturity
Amortized Cost	24,297		10,796
Allowance for credit losses on HTM	0		0
Net Carrying Amount	24,297		10,796
Gross Unrealized Gains	701		71
Gross Unrealized (Losses)	(1)		(59)
Estimated Fair Value	24,997		10,808
Available-for-sale
Amortized Cost	882,076		1,738,584
Allowance for credit losses on AFS	0		0
Gross Unrealized Gains	22,050		8,510
Gross Unrealized (Losses)	(439)		(4,149)
Estimated Fair Value	903,687		1,742,945
State and political subdivisions
Held-to-Maturity
Amortized Cost	22,003		27,082
Allowance for credit losses on HTM	(73)	(95)	0
Net Carrying Amount	21,930		27,082
Gross Unrealized Gains	1,155		849
Gross Unrealized (Losses)	(1)		0
Estimated Fair Value	23,084		27,931
Available-for-sale
Amortized Cost	1,105,341		860,539
Allowance for credit losses on AFS	(1,148)	(371)	0
Gross Unrealized Gains	32,001		20,983
Gross Unrealized (Losses)	(3,188)		(998)
Estimated Fair Value	1,133,006		880,524
Other securities
Held-to-Maturity
Amortized Cost	1,175		3,049
Allowance for credit losses on HTM	(300)	(212)	0
Net Carrying Amount	875		3,049
Gross Unrealized Gains	108		67
Gross Unrealized (Losses)			0
Estimated Fair Value	983		3,116
Available-for-sale
Amortized Cost	97,313		20,092
Allowance for credit losses on AFS	(60)	$ (238)	0
Gross Unrealized Gains	1,429		822
Gross Unrealized (Losses)	(60)		(18)
Estimated Fair Value	$ 98,622		$ 20,896