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Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($)
Jun. 30, 2018
Dec. 31, 2017
Securities available-for-sale [Abstract]    
Amortized Cost $ 3,008,104,000 $ 2,513,540,000
Gross Unrealized Gains 8,866,000 19,244,000
Gross Unrealized Losses 56,842,000 17,501,000
Securities available-for-sale, at fair value 2,960,128,000 2,515,283,000
Available-for-sale, Amortized Cost [Abstract]    
Due in one year or less 0  
Due in one year to five years 102,756,000  
Due in five years to ten years 176,890,000  
Due after ten years 1,218,580,000  
Mortgage-backed securities 1,269,307,000  
Asset-backed securities 240,571,000  
Amortized Cost 3,008,104,000  
Available-for-sale, Fair Value [Abstract]    
Due in one year or less 0  
Due in one year to five years 102,863,000  
Due in five years to ten years 175,583,000  
Due after ten years 1,202,205,000  
Mortgage-backed securities 1,239,031,000  
Asset-backed securities 240,446,000  
Securities available-for-sale, at fair value 2,960,128,000 2,515,283,000
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost 0  
Securities held-to-maturity [Abstract]    
Amortized Cost 15,341,000 20,762,000
Gross Unrealized Gains 31,000 114,000
Gross Unrealized Losses 113,000 46,000
Securities held-to-maturity, fair value 15,259,000 20,830,000
Held-to-maturity, Amortized Cost [Abstract]    
Due in one year to five years 6,412,000  
Due in five years to ten years 8,015,000  
Due after ten years 914,000  
Mortgage-backed securities 0  
Asset-backed securities 0  
Amortized Cost 15,341,000 20,762,000
Held-to-maturity, Fair Value [Abstract]    
Due in one year or less 0  
Due in one year to five years 6,405,000  
Due in five years to ten years 7,941,000  
Due after ten years 913,000  
Mortgage-backed securities 0  
Asset-backed securities 0  
Fair Value 15,259,000 20,830,000
U.S. Treasury securities    
Securities available-for-sale [Abstract]    
Amortized Cost 30,806,000 30,505,000
Gross Unrealized Gains 0 0
Gross Unrealized Losses 26,000 60,000
Securities available-for-sale, at fair value 30,780,000 30,445,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 30,780,000 30,445,000
U.S. government agency securities    
Securities available-for-sale [Abstract]    
Amortized Cost 177,178,000 182,500,000
Gross Unrealized Gains 37,000 67,000
Gross Unrealized Losses 3,986,000 1,766,000
Securities available-for-sale, at fair value 173,229,000 180,801,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 173,229,000 180,801,000
Mortgage-backed securities    
Securities available-for-sale [Abstract]    
Amortized Cost 1,269,307,000 1,270,625,000
Gross Unrealized Gains 3,866,000 5,318,000
Gross Unrealized Losses 34,142,000 12,124,000
Securities available-for-sale, at fair value 1,239,031,000 1,263,819,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 1,239,031,000 1,263,819,000
State and municipal securities    
Securities available-for-sale [Abstract]    
Amortized Cost 1,220,415,000 774,949,000
Gross Unrealized Gains 3,775,000 12,251,000
Gross Unrealized Losses 17,009,000 2,588,000
Securities available-for-sale, at fair value 1,207,181,000 784,612,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 1,207,181,000 784,612,000
Asset-backed securities    
Securities available-for-sale [Abstract]    
Amortized Cost 240,571,000 173,346,000
Gross Unrealized Gains 464,000 262,000
Gross Unrealized Losses 589,000 316,000
Securities available-for-sale, at fair value 240,446,000 173,292,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 240,446,000 173,292,000
Corporate notes and other    
Securities available-for-sale [Abstract]    
Amortized Cost 69,827,000 81,615,000
Gross Unrealized Gains 724,000 1,346,000
Gross Unrealized Losses 1,090,000 647,000
Securities available-for-sale, at fair value 69,461,000 82,314,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 69,461,000 82,314,000
State and municipal securities    
Securities held-to-maturity [Abstract]    
Amortized Cost 15,341,000 20,762,000
Gross Unrealized Gains 31,000 114,000
Gross Unrealized Losses 113,000 46,000
Securities held-to-maturity, fair value 15,259,000 20,830,000
Held-to-maturity, Amortized Cost [Abstract]    
Amortized Cost 15,341,000 20,762,000
Held-to-maturity, Fair Value [Abstract]    
Fair Value $ 15,259,000 $ 20,830,000