Securities- Unrealized Losses (Details) - USD ($) $ in Thousands	Mar. 31, 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	$ 365,407	$ 781,054
Investments with an unrealized loss of less than 12 months, unrealized losses	17,559	7,851
Investments with an unrealized loss of 12 months or longer, fair value	632,574	516,267
Investments with an unrealized loss of 12 months or longer, unrealized losses	45,350	9,934
Total investments with an unrealized loss, fair value	997,981	1,297,321
Total investments with an unrealized loss, unrealized losses	62,909	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	0	40,505
Investments with an unrealized loss of less than 12 months, unrealized losses	0	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	0	40,505
Total investments with an unrealized loss, unrealized losses	0	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	0	1,222
Investments with an unrealized loss of less than 12 months, unrealized losses	0	1
Investments with an unrealized loss of 12 months or longer, fair value	26,858	30,892
Investments with an unrealized loss of 12 months or longer, unrealized losses	1,072	709
Total investments with an unrealized loss, fair value	26,858	32,114
Total investments with an unrealized loss, unrealized losses	1,072	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	127,612	458,881
Investments with an unrealized loss of less than 12 months, unrealized losses	1,599	5,102
Investments with an unrealized loss of 12 months or longer, fair value	61,820	163,767
Investments with an unrealized loss of 12 months or longer, unrealized losses	1,337	2,674
Total investments with an unrealized loss, fair value	189,432	622,648
Total investments with an unrealized loss, unrealized losses	2,936	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	137,622	204,958
Investments with an unrealized loss of less than 12 months, unrealized losses	6,616	1,938
Investments with an unrealized loss of 12 months or longer, fair value	487,471	244,884
Investments with an unrealized loss of 12 months or longer, unrealized losses	35,555	5,311
Total investments with an unrealized loss, fair value	625,093	449,842
Total investments with an unrealized loss, unrealized losses	42,171	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	80,904	75,488
Investments with an unrealized loss of less than 12 months, unrealized losses	7,905	796
Investments with an unrealized loss of 12 months or longer, fair value	48,125	59,816
Investments with an unrealized loss of 12 months or longer, unrealized losses	5,686	497
Total investments with an unrealized loss, fair value	129,029	135,304
Total investments with an unrealized loss, unrealized losses	13,591	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	19,269	0
Investments with an unrealized loss of less than 12 months, unrealized losses	1,439	0
Investments with an unrealized loss of 12 months or longer, fair value	8,300	16,908
Investments with an unrealized loss of 12 months or longer, unrealized losses	1,700	743
Total investments with an unrealized loss, fair value	27,569	16,908
Total investments with an unrealized loss, unrealized losses	$ 3,139	$ 743