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Securities- Unrealized Losses (Details) - USD ($)
$ in Thousands
Mar. 31, 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 $ 1,930,176 $ 1,017,346
Investments with an unrealized loss of less than 12 months, unrealized losses 37,542 9,021
Investments with an unrealized loss of 12 months or longer, fair value 367,056 361,724
Investments with an unrealized loss of 12 months or longer, unrealized losses 14,543 8,526
Total investments with an unrealized loss, fair value 2,297,232 1,379,070
Total investments with an unrealized loss, unrealized losses 52,085 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 30,697 29,948
Investments with an unrealized loss of less than 12 months, unrealized losses 56 60
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 30,697 29,948
Total investments with an unrealized loss, unrealized losses 56 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 145,967 173,677
Investments with an unrealized loss of less than 12 months, unrealized losses 2,904 1,766
Investments with an unrealized loss of 12 months or longer, fair value 248 0
Investments with an unrealized loss of 12 months or longer, unrealized losses 1 0
Total investments with an unrealized loss, fair value 146,215 173,677
Total investments with an unrealized loss, unrealized losses 2,905 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 779,194 607,408
Investments with an unrealized loss of less than 12 months, unrealized losses 17,392 5,042
Investments with an unrealized loss of 12 months or longer, fair value 297,735 285,561
Investments with an unrealized loss of 12 months or longer, unrealized losses 12,002 7,082
Total investments with an unrealized loss, fair value 1,076,929 892,969
Total investments with an unrealized loss, unrealized losses 29,394 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 863,010 115,403
Investments with an unrealized loss of less than 12 months, unrealized losses 15,987 1,408
Investments with an unrealized loss of 12 months or longer, fair value 47,894 50,083
Investments with an unrealized loss of 12 months or longer, unrealized losses 2,335 1,226
Total investments with an unrealized loss, fair value 910,904 165,486
Total investments with an unrealized loss, unrealized losses 18,322 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 79,428 68,742
Investments with an unrealized loss of less than 12 months, unrealized losses 449 198
Investments with an unrealized loss of 12 months or longer, fair value 9,482 14,136
Investments with an unrealized loss of 12 months or longer, unrealized losses 56 118
Total investments with an unrealized loss, fair value 88,910 82,878
Total investments with an unrealized loss, unrealized losses 505 316
Corporate notes    
Available-for-sale securities, continuous unrealized loss position [Abstract]    
Investments with an unrealized loss of less than 12 months, fair value 31,880 22,168
Investments with an unrealized loss of less than 12 months, unrealized losses 754 547
Investments with an unrealized loss of 12 months or longer, fair value 11,697 11,944
Investments with an unrealized loss of 12 months or longer, unrealized losses 149 100
Total investments with an unrealized loss, fair value 43,577 34,112
Total investments with an unrealized loss, unrealized losses $ 903 $ 647