Securities- Unrealized Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	$ 475,732	$ 781,054
Investments with an unrealized loss of less than 12 months, unrealized losses	5,889	7,851
Investments with an unrealized loss of 12 months or longer, fair value	675,694	516,267
Investments with an unrealized loss of 12 months or longer, unrealized losses	29,399	9,934
Total investments with an unrealized loss, fair value	1,151,426	1,297,321
Total investments with an unrealized loss, unrealized losses	35,288	17,785
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	25,973	40,505
Investments with an unrealized loss of less than 12 months, unrealized losses	2	14
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	25,973	40,505
Total investments with an unrealized loss, unrealized losses	2	14
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	10,576	1,222
Investments with an unrealized loss of less than 12 months, unrealized losses	4	1
Investments with an unrealized loss of 12 months or longer, fair value	6,360	30,892
Investments with an unrealized loss of 12 months or longer, unrealized losses	25	709
Total investments with an unrealized loss, fair value	16,936	32,114
Total investments with an unrealized loss, unrealized losses	29	710
Mortgage-backed securities
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	138,532	458,881
Investments with an unrealized loss of less than 12 months, unrealized losses	1,746	5,102
Investments with an unrealized loss of 12 months or longer, fair value	39,350	163,767
Investments with an unrealized loss of 12 months or longer, unrealized losses	682	2,674
Total investments with an unrealized loss, fair value	177,882	622,648
Total investments with an unrealized loss, unrealized losses	2,428	7,776
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	199,365	204,958
Investments with an unrealized loss of less than 12 months, unrealized losses	3,608	1,938
Investments with an unrealized loss of 12 months or longer, fair value	539,131	244,884
Investments with an unrealized loss of 12 months or longer, unrealized losses	24,676	5,311
Total investments with an unrealized loss, fair value	738,496	449,842
Total investments with an unrealized loss, unrealized losses	28,284	7,249
Asset-backed securities
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	74,977	75,488
Investments with an unrealized loss of less than 12 months, unrealized losses	355	796
Investments with an unrealized loss of 12 months or longer, fair value	67,973	59,816
Investments with an unrealized loss of 12 months or longer, unrealized losses	816	497
Total investments with an unrealized loss, fair value	142,950	135,304
Total investments with an unrealized loss, unrealized losses	1,171	1,293
Corporate notes
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	26,309	0
Investments with an unrealized loss of less than 12 months, unrealized losses	174	0
Investments with an unrealized loss of 12 months or longer, fair value	22,880	16,908
Investments with an unrealized loss of 12 months or longer, unrealized losses	3,200	743
Total investments with an unrealized loss, fair value	49,189	16,908
Total investments with an unrealized loss, unrealized losses	$ 3,374	$ 743