Securities- Unrealized Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	$ 1,704,605	$ 1,017,346
Investments with an unrealized loss of less than 12 months, unrealized losses	36,950	9,021
Investments with an unrealized loss of 12 months or longer, fair value	984,619	361,724
Investments with an unrealized loss of 12 months or longer, unrealized losses	44,939	8,526
Total investments with an unrealized loss, fair value	2,689,224	1,379,070
Total investments with an unrealized loss, unrealized losses	81,889	17,547
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	30,284	29,948
Investments with an unrealized loss of less than 12 months, unrealized losses	39	60
Investments with an unrealized loss of 12 months or longer, fair value	244	0
Investments with an unrealized loss of 12 months or longer, unrealized losses	6	0
Total investments with an unrealized loss, fair value	30,528	29,948
Total investments with an unrealized loss, unrealized losses	45	60
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	13,605	173,677
Investments with an unrealized loss of less than 12 months, unrealized losses	360	1,766
Investments with an unrealized loss of 12 months or longer, fair value	45,118	0
Investments with an unrealized loss of 12 months or longer, unrealized losses	1,140	0
Total investments with an unrealized loss, fair value	58,723	173,677
Total investments with an unrealized loss, unrealized losses	1,500	1,766
Mortgage-backed securities
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	289,149	607,408
Investments with an unrealized loss of less than 12 months, unrealized losses	5,951	5,042
Investments with an unrealized loss of 12 months or longer, fair value	807,418	285,561
Investments with an unrealized loss of 12 months or longer, unrealized losses	37,412	7,082
Total investments with an unrealized loss, fair value	1,096,567	892,969
Total investments with an unrealized loss, unrealized losses	43,363	12,124
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	1,144,600	115,403
Investments with an unrealized loss of less than 12 months, unrealized losses	29,599	1,408
Investments with an unrealized loss of 12 months or longer, fair value	108,653	50,083
Investments with an unrealized loss of 12 months or longer, unrealized losses	5,663	1,226
Total investments with an unrealized loss, fair value	1,253,253	165,486
Total investments with an unrealized loss, unrealized losses	35,262	2,634
Asset-backed securities
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	204,638	68,742
Investments with an unrealized loss of less than 12 months, unrealized losses	741	198
Investments with an unrealized loss of 12 months or longer, fair value	4,505	14,136
Investments with an unrealized loss of 12 months or longer, unrealized losses	18	118
Total investments with an unrealized loss, fair value	209,143	82,878
Total investments with an unrealized loss, unrealized losses	759	316
Corporate notes
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	22,329	22,168
Investments with an unrealized loss of less than 12 months, unrealized losses	260	547
Investments with an unrealized loss of 12 months or longer, fair value	18,681	11,944
Investments with an unrealized loss of 12 months or longer, unrealized losses	700	100
Total investments with an unrealized loss, fair value	41,010	34,112
Total investments with an unrealized loss, unrealized losses	$ 960	$ 647