Securities - Amortized cost, gross unrealized gains and losses, and fair value of debt securities available-for-sale (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt securities available for sale and equity securities:
Amortized Cost	$ 2,202,217	$ 2,240,607
Gross Unrealized Gains	57,788	36,982
Gross Unrealized Losses	10,129	3,008
Fair Value	2,249,876	2,274,581
U.S. Treasuries
Debt securities available for sale and equity securities:
Amortized Cost	153,219	152,990
Gross Unrealized Gains	5,745	1,628
Gross Unrealized Losses	0	0
Fair Value	158,964	154,618
U.S. Government agencies
Debt securities available for sale and equity securities:
Amortized Cost	2,777	2,848
Gross Unrealized Gains	190	188
Gross Unrealized Losses	0	1
Fair Value	2,967	3,035
State and political subdivisions
Debt securities available for sale and equity securities:
Amortized Cost	214,191	214,677
Gross Unrealized Gains	13,507	11,813
Gross Unrealized Losses	17	0
Fair Value	227,681	226,490
Residential mortgage-backed securities, Agency
Debt securities available for sale and equity securities:
Amortized Cost	1,008,944	1,030,948
Gross Unrealized Gains	29,264	12,022
Gross Unrealized Losses	459	726
Fair Value	1,037,749	1,042,244
Residential mortgage-backed securities, Non-agency
Debt securities available for sale and equity securities:
Amortized Cost	260,638	250,550
Gross Unrealized Gains	2,095	6,231
Gross Unrealized Losses	2,891	0
Fair Value	259,842	256,781
Commercial mortgage-backed, Agency
Debt securities available for sale and equity securities:
Amortized Cost	261,836	266,770
Gross Unrealized Gains	5,982	2,261
Gross Unrealized Losses	29	128
Fair Value	267,789	268,903
Commercial mortgage-backed, Non-agency
Debt securities available for sale and equity securities:
Amortized Cost		15,395
Gross Unrealized Gains		918
Gross Unrealized Losses		263
Fair Value		16,050
Corporate bonds
Debt securities available for sale and equity securities:
Amortized Cost	191,070	202,131
Gross Unrealized Gains	665	1,178
Gross Unrealized Losses	950	218
Fair Value	190,785	203,091
Asset-backed securities
Debt securities available for sale and equity securities:
Amortized Cost	109,542	104,298
Gross Unrealized Gains	340	743
Gross Unrealized Losses	5,783	1,672
Fair Value	$ 104,099	$ 103,369

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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