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Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($)
 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Securities available-for-sale [Abstract]      
Amortized Cost $ 3,074,274,000   $ 2,513,540,000
Gross Unrealized Gains 6,795,000   19,244,000
Gross Unrealized Losses 76,487,000   17,501,000
Securities available-for-sale, at fair value 3,004,582,000   2,515,283,000
Available-for-sale, Amortized Cost [Abstract]      
Due in one year or less 34,856,000    
Due in one year to five years 77,055,000    
Due in five years to ten years 159,471,000    
Due after ten years 1,133,705,000    
Mortgage-backed securities 1,334,640,000    
Asset-backed securities 334,547,000    
Amortized Cost 3,074,274,000    
Available-for-sale, Fair Value [Abstract]      
Due in one year or less 34,802,000    
Due in one year to five years 77,059,000    
Due in five years to ten years 157,274,000    
Due after ten years 1,106,244,000    
Mortgage-backed securities 1,294,545,000    
Asset-backed securities 334,658,000    
Securities available-for-sale, at fair value 3,004,582,000   2,515,283,000
Schedule of Held-to-maturity Securities [Line Items]      
Available-for-sale securities transferred to Held-to-Maturity 179,763,000 $ 0  
Unrealized after tax loss on available-for-sale securities transferred to the held-to-maturity portfolio 2,200,000    
Securities held-to-maturity [Abstract]      
Amortized Cost 194,997,000   20,762,000
Gross Unrealized Gains 9,000   114,000
Gross Unrealized Losses 2,506,000   46,000
Securities held-to-maturity, fair value 192,500,000   20,830,000
Held-to-maturity, Amortized Cost [Abstract]      
Due in one year or less 315,000    
Due in one year to five years 6,066,000    
Due in five years to ten years 7,997,000    
Due after ten years 180,619,000    
Mortgage-backed securities 0    
Asset-backed securities 0    
Amortized Cost 194,997,000   20,762,000
Held-to-maturity, Fair Value [Abstract]      
Due in one year or less 316,000    
Due in one year to five years 6,044,000    
Due in five years to ten years 7,875,000    
Due after ten years 178,265,000    
Mortgage-backed securities 0    
Asset-backed securities 0    
Fair Value 192,500,000   20,830,000
U.S. Treasury securities      
Securities available-for-sale [Abstract]      
Amortized Cost 30,573,000   30,505,000
Gross Unrealized Gains 0   0
Gross Unrealized Losses 45,000   60,000
Securities available-for-sale, at fair value 30,528,000   30,445,000
Available-for-sale, Fair Value [Abstract]      
Securities available-for-sale, at fair value 30,528,000   30,445,000
U.S. government agency securities      
Securities available-for-sale [Abstract]      
Amortized Cost 64,640,000   182,500,000
Gross Unrealized Gains 45,000   67,000
Gross Unrealized Losses 1,500,000   1,766,000
Securities available-for-sale, at fair value 63,185,000   180,801,000
Available-for-sale, Fair Value [Abstract]      
Securities available-for-sale, at fair value 63,185,000   180,801,000
Mortgage-backed securities      
Securities available-for-sale [Abstract]      
Amortized Cost 1,334,640,000   1,270,625,000
Gross Unrealized Gains 3,268,000   5,318,000
Gross Unrealized Losses 43,363,000   12,124,000
Securities available-for-sale, at fair value 1,294,545,000   1,263,819,000
Available-for-sale, Fair Value [Abstract]      
Securities available-for-sale, at fair value 1,294,545,000   1,263,819,000
State and municipal securities      
Securities available-for-sale [Abstract]      
Amortized Cost 1,240,136,000   774,949,000
Gross Unrealized Gains 1,995,000   12,251,000
Gross Unrealized Losses 29,860,000   2,588,000
Securities available-for-sale, at fair value 1,212,271,000   784,612,000
Available-for-sale, Fair Value [Abstract]      
Securities available-for-sale, at fair value 1,212,271,000   784,612,000
Asset-backed securities      
Securities available-for-sale [Abstract]      
Amortized Cost 334,547,000   173,346,000
Gross Unrealized Gains 870,000   262,000
Gross Unrealized Losses 759,000   316,000
Securities available-for-sale, at fair value 334,658,000   173,292,000
Available-for-sale, Fair Value [Abstract]      
Securities available-for-sale, at fair value 334,658,000   173,292,000
Corporate notes and other      
Securities available-for-sale [Abstract]      
Amortized Cost 69,738,000   81,615,000
Gross Unrealized Gains 617,000   1,346,000
Gross Unrealized Losses 960,000   647,000
Securities available-for-sale, at fair value 69,395,000   82,314,000
Available-for-sale, Fair Value [Abstract]      
Securities available-for-sale, at fair value 69,395,000   82,314,000
State and municipal securities      
Securities held-to-maturity [Abstract]      
Amortized Cost 194,997,000   20,762,000
Gross Unrealized Gains 9,000   114,000
Gross Unrealized Losses 2,506,000   46,000
Securities held-to-maturity, fair value 192,500,000   20,830,000
Held-to-maturity, Amortized Cost [Abstract]      
Amortized Cost 194,997,000   20,762,000
Held-to-maturity, Fair Value [Abstract]      
Fair Value $ 192,500,000   $ 20,830,000