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Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($)
Mar. 31, 2018
Dec. 31, 2017
Securities available-for-sale [Abstract]    
Amortized Cost $ 3,003,018,000 $ 2,513,540,000
Gross Unrealized Gains 9,571,000 19,244,000
Gross Unrealized Losses 51,965,000 17,501,000
Securities available-for-sale, at fair value 2,960,624,000 2,515,283,000
Available-for-sale, Amortized Cost [Abstract]    
Due in one year or less 0  
Due in one year to five years 96,941,000  
Due in five years to ten years 177,898,000  
Due after ten years 1,221,105,000  
Mortgage-backed securities 1,302,274,000  
Asset-backed securities 204,800,000  
Amortized Cost 3,003,018,000  
Available-for-sale, Fair Value [Abstract]    
Due in one year or less 0  
Due in one year to five years 97,087,000  
Due in five years to ten years 177,231,000  
Due after ten years 1,204,361,000  
Mortgage-backed securities 1,277,273,000  
Asset-backed securities 204,672,000  
Securities available-for-sale, at fair value 2,960,624,000 2,515,283,000
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount 0  
Securities held-to-maturity [Abstract]    
Amortized Cost 20,677,000 20,762,000
Gross Unrealized Gains 46,000 114,000
Gross Unrealized Losses 120,000 46,000
Securities held-to-maturity, fair value 20,603,000 20,830,000
Held-to-maturity, Amortized Cost [Abstract]    
Due in one year to five years 7,496,000  
Due in five years to ten years 10,385,000  
Due after ten years 2,796,000  
Mortgage-backed securities 0  
Asset-backed securities 0  
Amortized Cost 20,677,000 20,762,000
Held-to-maturity, Fair Value [Abstract]    
Due in one year or less 0  
Due in one year to five years 7,489,000  
Due in five years to ten years 10,316,000  
Due after ten years 2,798,000  
Mortgage-backed securities 0  
Asset-backed securities 0  
Fair Value 20,603,000 20,830,000
U.S. Treasury securities    
Securities available-for-sale [Abstract]    
Amortized Cost 31,002,000 30,505,000
Gross Unrealized Gains 0 0
Gross Unrealized Losses 56,000 60,000
Securities available-for-sale, at fair value 30,946,000 30,445,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 30,946,000 30,445,000
U.S. government agency securities    
Securities available-for-sale [Abstract]    
Amortized Cost 176,305,000 182,500,000
Gross Unrealized Gains 43,000 67,000
Gross Unrealized Losses 2,905,000 1,766,000
Securities available-for-sale, at fair value 173,443,000 180,801,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 173,443,000 180,801,000
Mortgage-backed securities    
Securities available-for-sale [Abstract]    
Amortized Cost 1,302,274,000 1,270,625,000
Gross Unrealized Gains 4,393,000 5,318,000
Gross Unrealized Losses 29,394,000 12,124,000
Securities available-for-sale, at fair value 1,277,273,000 1,263,819,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 1,277,273,000 1,263,819,000
State and municipal securities    
Securities available-for-sale [Abstract]    
Amortized Cost 1,206,777,000 774,949,000
Gross Unrealized Gains 3,890,000 12,251,000
Gross Unrealized Losses 18,202,000 2,588,000
Securities available-for-sale, at fair value 1,192,465,000 784,612,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 1,192,465,000 784,612,000
Asset-backed securities    
Securities available-for-sale [Abstract]    
Amortized Cost 204,800,000 173,346,000
Gross Unrealized Gains 377,000 262,000
Gross Unrealized Losses 505,000 316,000
Securities available-for-sale, at fair value 204,672,000 173,292,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 204,672,000 173,292,000
Corporate notes and other    
Securities available-for-sale [Abstract]    
Amortized Cost 81,860,000 81,615,000
Gross Unrealized Gains 868,000 1,346,000
Gross Unrealized Losses 903,000 647,000
Securities available-for-sale, at fair value 81,825,000 82,314,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 81,825,000 82,314,000
State and municipal securities    
Securities held-to-maturity [Abstract]    
Amortized Cost 20,677,000 20,762,000
Gross Unrealized Gains 46,000 114,000
Gross Unrealized Losses 120,000 46,000
Securities held-to-maturity, fair value 20,603,000 20,830,000
Held-to-maturity, Amortized Cost [Abstract]    
Amortized Cost 20,677,000 20,762,000
Held-to-maturity, Fair Value [Abstract]    
Fair Value $ 20,603,000 $ 20,830,000