Securities- Unrealized Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	$ 901,524	$ 407,150
Investments with an unrealized loss of less than 12 months, unrealized losses	23,235	4,277
Investments with an unrealized loss of 12 months or longer, fair value	328,449	464,283
Investments with an unrealized loss of 12 months or longer, unrealized losses	6,662	13,560
Total investments with an unrealized loss, fair value	1,229,973	871,433
Total investments with an unrealized loss, unrealized losses	29,897	17,837
U.S. Treasury securities
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	0	0
Investments with an unrealized loss of less than 12 months, unrealized losses	0	0
Investments with an unrealized loss of 12 months or longer, fair value	0	0
Investments with an unrealized loss of 12 months or longer, unrealized losses	0	0
Total investments with an unrealized loss, fair value	0	0
Total investments with an unrealized loss, unrealized losses	0	0
U.S. government agency securities
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	109,710	9,962
Investments with an unrealized loss of less than 12 months, unrealized losses	3,355	38
Investments with an unrealized loss of 12 months or longer, fair value	5,701	6,091
Investments with an unrealized loss of 12 months or longer, unrealized losses	26	22
Total investments with an unrealized loss, fair value	115,411	16,053
Total investments with an unrealized loss, unrealized losses	3,381	60
Mortgage-backed securities
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	427,992	165,696
Investments with an unrealized loss of less than 12 months, unrealized losses	14,434	1,772
Investments with an unrealized loss of 12 months or longer, fair value	30,072	35,997
Investments with an unrealized loss of 12 months or longer, unrealized losses	379	555
Total investments with an unrealized loss, fair value	458,064	201,693
Total investments with an unrealized loss, unrealized losses	14,813	2,327
State and municipal securities
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	287,208	175,115
Investments with an unrealized loss of less than 12 months, unrealized losses	4,779	2,220
Investments with an unrealized loss of 12 months or longer, fair value	253,998	345,435
Investments with an unrealized loss of 12 months or longer, unrealized losses	4,379	10,264
Total investments with an unrealized loss, fair value	541,206	520,550
Total investments with an unrealized loss, unrealized losses	9,158	12,484
Asset-backed securities
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	62,619	46,399
Investments with an unrealized loss of less than 12 months, unrealized losses	662	207
Investments with an unrealized loss of 12 months or longer, fair value	13,228	52,840
Investments with an unrealized loss of 12 months or longer, unrealized losses	30	450
Total investments with an unrealized loss, fair value	75,847	99,239
Total investments with an unrealized loss, unrealized losses	692	657
Corporate notes
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	13,995	9,978
Investments with an unrealized loss of less than 12 months, unrealized losses	5	40
Investments with an unrealized loss of 12 months or longer, fair value	25,450	23,920
Investments with an unrealized loss of 12 months or longer, unrealized losses	1,848	2,269
Total investments with an unrealized loss, fair value	39,445	33,898
Total investments with an unrealized loss, unrealized losses	$ 1,853	$ 2,309