Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Held-to-Maturity
Amortized Cost	$ 52,027		$ 40,927
Allowance for credit losses on HTM	(307)	$ (409)	0
Net Carrying Amount	51,720		40,927
Gross Unrealized Gains	2,034		987
Gross Unrealized (Losses)	(3)		(59)
Estimated Fair Value	53,751		41,855
Available-for-sale
Amortized Cost	2,428,548		3,263,151
Allowance for credit losses on AFS	(609)	(269)	0
Gross Unrealized Gains	71,084		31,740
Gross Unrealized (Losses)	(2,127)		(6,548)
Estimated Fair Value	2,496,896		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	210,496		194,207
Allowance for credit losses on AFS	0		0
Gross Unrealized Gains	1,416		1,313
Gross Unrealized (Losses)	(991)		(1,271)
Estimated Fair Value	210,921		194,249
Mortgage-backed securities
Held-to-Maturity
Amortized Cost	25,980		10,796
Allowance for credit losses on HTM	0		0
Net Carrying Amount	25,980		10,796
Gross Unrealized Gains	798		71
Gross Unrealized (Losses)	(1)		(59)
Estimated Fair Value	26,777		10,808
Available-for-sale
Amortized Cost	1,125,484		1,738,584
Allowance for credit losses on AFS	0		0
Gross Unrealized Gains	28,804		8,510
Gross Unrealized (Losses)	(202)		(4,149)
Estimated Fair Value	1,154,086		1,742,945
State and political subdivisions
Held-to-Maturity
Amortized Cost	24,872		27,082
Allowance for credit losses on HTM	(95)	(97)	0
Net Carrying Amount	24,777		27,082
Gross Unrealized Gains	1,125		849
Gross Unrealized (Losses)	(2)		0
Estimated Fair Value	25,900		27,931
Available-for-sale
Amortized Cost	1,015,625		860,539
Allowance for credit losses on AFS	(371)	(95)	0
Gross Unrealized Gains	39,510		20,983
Gross Unrealized (Losses)	(696)		(998)
Estimated Fair Value	1,054,068		880,524
Other securities
Held-to-Maturity
Amortized Cost	1,175		3,049
Allowance for credit losses on HTM	(212)	(312)	0
Net Carrying Amount	963		3,049
Gross Unrealized Gains	111		67
Gross Unrealized (Losses)	0		0
Estimated Fair Value	1,074		3,116
Available-for-sale
Amortized Cost	76,943		20,092
Allowance for credit losses on AFS	(238)	$ (174)	0
Gross Unrealized Gains	1,354		822
Gross Unrealized (Losses)	(238)		(18)
Estimated Fair Value	$ 77,821		$ 20,896

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