Securities- Unrealized Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	$ 443,073	$ 1,347,820
Investments with an unrealized loss of less than 12 months, unrealized losses	5,501	26,783
Investments with an unrealized loss of 12 months or longer, fair value	1,166,184	1,161,369
Investments with an unrealized loss of 12 months or longer, unrealized losses	18,593	32,811
Total investments with an unrealized loss, fair value	1,609,257	2,509,189
Total investments with an unrealized loss, unrealized losses	24,094	59,594
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	29,824	30,054
Investments with an unrealized loss of less than 12 months, unrealized losses	3	22
Investments with an unrealized loss of 12 months or longer, fair value	248	246
Investments with an unrealized loss of 12 months or longer, unrealized losses	2	3
Total investments with an unrealized loss, fair value	30,072	30,300
Total investments with an unrealized loss, unrealized losses	5	25
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	6,631	13,697
Investments with an unrealized loss of less than 12 months, unrealized losses	58	328
Investments with an unrealized loss of 12 months or longer, fair value	51,280	42,539
Investments with an unrealized loss of 12 months or longer, unrealized losses	1,217	1,018
Total investments with an unrealized loss, fair value	57,911	56,236
Total investments with an unrealized loss, unrealized losses	1,275	1,346
Mortgage-backed securities
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	82,112	203,299
Investments with an unrealized loss of less than 12 months, unrealized losses	565	2,134
Investments with an unrealized loss of 12 months or longer, fair value	987,424	882,231
Investments with an unrealized loss of 12 months or longer, unrealized losses	15,760	26,500
Total investments with an unrealized loss, fair value	1,069,536	1,085,530
Total investments with an unrealized loss, unrealized losses	16,325	28,634
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	161,772	805,821
Investments with an unrealized loss of less than 12 months, unrealized losses	2,622	18,643
Investments with an unrealized loss of 12 months or longer, fair value	80,686	198,610
Investments with an unrealized loss of 12 months or longer, unrealized losses	509	4,078
Total investments with an unrealized loss, fair value	242,458	1,004,431
Total investments with an unrealized loss, unrealized losses	3,131	22,721
Asset-backed securities
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	152,062	268,677
Investments with an unrealized loss of less than 12 months, unrealized losses	1,534	4,118
Investments with an unrealized loss of 12 months or longer, fair value	26,675	11,828
Investments with an unrealized loss of 12 months or longer, unrealized losses	423	227
Total investments with an unrealized loss, fair value	178,737	280,505
Total investments with an unrealized loss, unrealized losses	1,957	4,345
Corporate notes
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	10,672	26,272
Investments with an unrealized loss of less than 12 months, unrealized losses	719	1,538
Investments with an unrealized loss of 12 months or longer, fair value	19,871	25,915
Investments with an unrealized loss of 12 months or longer, unrealized losses	682	985
Total investments with an unrealized loss, fair value	30,543	52,187
Total investments with an unrealized loss, unrealized losses	$ 1,401	$ 2,523