Securities- Unrealized Losses (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	$ 1,647,237	$ 907,810
Investments with an unrealized loss of less than 12 months, unrealized losses	17,749	18,219
Investments with an unrealized loss of 12 months or longer, fair value	138,421	98,377
Investments with an unrealized loss of 12 months or longer, unrealized losses	2,974	1,968
Total investments with an unrealized loss, fair value	1,785,658	1,006,187
Total investments with an unrealized loss, unrealized losses	20,723	20,187
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	49,120	0
Investments with an unrealized loss of less than 12 months, unrealized losses	536	0
Investments with an unrealized loss of 12 months or longer, fair value	12,780	20,820
Investments with an unrealized loss of 12 months or longer, unrealized losses	223	537
Total investments with an unrealized loss, fair value	61,900	20,820
Total investments with an unrealized loss, unrealized losses	759	537
Mortgage-backed securities
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	1,097,974	801,213
Investments with an unrealized loss of less than 12 months, unrealized losses	13,490	15,073
Investments with an unrealized loss of 12 months or longer, fair value	85,658	43,148
Investments with an unrealized loss of 12 months or longer, unrealized losses	1,658	613
Total investments with an unrealized loss, fair value	1,183,632	844,361
Total investments with an unrealized loss, unrealized losses	15,148	15,686
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	324,824	87,277
Investments with an unrealized loss of less than 12 months, unrealized losses	3,379	3,068
Investments with an unrealized loss of 12 months or longer, fair value	20,830	312
Investments with an unrealized loss of 12 months or longer, unrealized losses	776	1
Total investments with an unrealized loss, fair value	345,654	87,589
Total investments with an unrealized loss, unrealized losses	4,155	3,069
Asset-backed securities
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	113,296	14,510
Investments with an unrealized loss of less than 12 months, unrealized losses	157	32
Investments with an unrealized loss of 12 months or longer, fair value	19,153	34,097
Investments with an unrealized loss of 12 months or longer, unrealized losses	317	817
Total investments with an unrealized loss, fair value	132,449	48,607
Total investments with an unrealized loss, unrealized losses	474	849
Corporate notes
Available-for-sale securities, continuous unrealized loss position [Abstract]
Investments with an unrealized loss of less than 12 months, fair value	62,023	4,810
Investments with an unrealized loss of less than 12 months, unrealized losses	187	46
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	62,023	4,810
Total investments with an unrealized loss, unrealized losses	$ 187	$ 46