Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Held-to-Maturity
Amortized Cost	$ 54,377	$ 40,927
Allowance for credit losses on HTM	(409)	0
Net Carrying Amount	53,968	40,927
Gross Unrealized Gains	1,759	987
Gross Unrealized (Losses)	(13)	(59)
Estimated Fair Value	55,714	41,855
Available-for-sale
Amortized Cost	2,418,498	3,263,151
Allowance for credit losses on AFS	(269)	0
Gross Unrealized Gains	53,884	31,740
Gross Unrealized (Losses)	(5,473)	(6,548)
Estimated Fair Value	2,466,640	3,288,343
U.S. Treasury
Available-for-sale
Amortized Cost	425,032	449,729
Allowance for credit losses on AFS	0	0
Gross Unrealized Gains	0	112
Gross Unrealized (Losses)	(43)	(112)
Estimated Fair Value	424,989	449,729
U.S. Government agencies
Available-for-sale
Amortized Cost	161,473	194,207
Allowance for credit losses on AFS	0	0
Gross Unrealized Gains	707	1,313
Gross Unrealized (Losses)	(891)	(1,271)
Estimated Fair Value	161,289	194,249
Mortgage-backed securities
Held-to-Maturity
Amortized Cost	27,121	10,796
Allowance for credit losses on HTM	0	0
Net Carrying Amount	27,121	10,796
Gross Unrealized Gains	668	71
Gross Unrealized (Losses)	(1)	(59)
Estimated Fair Value	27,788	10,808
Available-for-sale
Amortized Cost	1,150,878	1,738,584
Allowance for credit losses on AFS	0	0
Gross Unrealized Gains	32,068	8,510
Gross Unrealized (Losses)	(3,109)	(4,149)
Estimated Fair Value	1,179,837	1,742,945
State and political subdivisions
Held-to-Maturity
Amortized Cost	26,082	27,082
Allowance for credit losses on HTM	(97)	0
Net Carrying Amount	25,985	27,082
Gross Unrealized Gains	981	849
Gross Unrealized (Losses)	(12)	0
Estimated Fair Value	26,954	27,931
Available-for-sale
Amortized Cost	659,350	860,539
Allowance for credit losses on AFS	(95)	0
Gross Unrealized Gains	20,243	20,983
Gross Unrealized (Losses)	(1,255)	(998)
Estimated Fair Value	678,243	880,524
Other securities
Held-to-Maturity
Amortized Cost	1,174	3,049
Allowance for credit losses on HTM	(312)	0
Net Carrying Amount	862	3,049
Gross Unrealized Gains	110	67
Gross Unrealized (Losses)	0	0
Estimated Fair Value	972	3,116
Available-for-sale
Amortized Cost	21,765	20,092
Allowance for credit losses on AFS	(174)	0
Gross Unrealized Gains	866	822
Gross Unrealized (Losses)	(175)	(18)
Estimated Fair Value	$ 22,282	$ 20,896

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