Securities - Amortized cost, gross unrealized gains and losses, and fair value of debt securities available-for-sale (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt securities available for sale:
Amortized Cost	$ 2,613,609	$ 2,240,607
Gross Unrealized Gains	79,136	36,982
Gross Unrealized Losses	2,297	3,008
Debt securities available-for-sale, fair value	2,690,448	2,274,581
U.S. Treasuries
Debt securities available for sale:
Amortized Cost	123,534	152,990
Gross Unrealized Gains	5,000	1,628
Gross Unrealized Losses	0	0
Debt securities available-for-sale, fair value	128,534	154,618
U.S. Government agencies
Debt securities available for sale:
Amortized Cost	51,312	2,848
Gross Unrealized Gains	491	188
Gross Unrealized Losses	58	1
Debt securities available-for-sale, fair value	51,745	3,035
State and political subdivisions
Debt securities available for sale:
Amortized Cost	239,190	214,677
Gross Unrealized Gains	19,999	11,813
Gross Unrealized Losses	13	0
Debt securities available-for-sale, fair value	259,176	226,490
Residential mortgage-backed securities, Agency
Debt securities available for sale:
Amortized Cost	1,038,637	1,030,948
Gross Unrealized Gains	31,772	12,022
Gross Unrealized Losses	276	726
Debt securities available-for-sale, fair value	1,070,133	1,042,244
Residential mortgage-backed securities, Non-agency
Debt securities available for sale:
Amortized Cost	218,097	250,550
Gross Unrealized Gains	11,574	6,231
Gross Unrealized Losses	0	0
Debt securities available-for-sale, fair value	229,671	256,781
Commercial mortgage-backed, Agency
Debt securities available for sale:
Amortized Cost	386,340	266,770
Gross Unrealized Gains	8,254	2,261
Gross Unrealized Losses	77	128
Debt securities available-for-sale, fair value	394,517	268,903
Commercial mortgage-backed, Non-agency
Debt securities available for sale:
Amortized Cost		15,395
Gross Unrealized Gains		918
Gross Unrealized Losses		263
Debt securities available-for-sale, fair value		16,050
Corporate bonds
Debt securities available for sale:
Amortized Cost	97,758	202,131
Gross Unrealized Gains	1,537	1,178
Gross Unrealized Losses	25	218
Debt securities available-for-sale, fair value	99,270	203,091
Asset-backed securities
Debt securities available for sale:
Amortized Cost	458,741	104,298
Gross Unrealized Gains	509	743
Gross Unrealized Losses	1,848	1,672
Debt securities available-for-sale, fair value	$ 457,402	$ 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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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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