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Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($)
Mar. 31, 2021
Dec. 31, 2020
Securities available-for-sale [Abstract]    
Amortized Cost $ 3,620,716,000 $ 3,487,296,000
Gross Unrealized Gains 85,513,000 117,222,000
Gross Unrealized Losses 29,210,000 17,837,000
Securities available-for-sale, at fair value 3,677,019,000 3,586,681,000
Available-for-sale, Amortized Cost [Abstract]    
Due in one year or less 88,478,000  
Due in one year to five years 13,022,000  
Due in five years to ten years 265,259,000  
Due after ten years 1,479,282,000  
Mortgage-backed securities 1,583,670,000  
Asset-backed securities 191,005,000  
Amortized Cost 3,620,716,000  
Available-for-sale, Fair Value [Abstract]    
Due in one year or less 88,502,000  
Due in one year to five years 13,266,000  
Due in five years to ten years 278,713,000  
Due after ten years 1,496,065,000  
Mortgage-backed securities 1,609,778,000  
Asset-backed securities 190,695,000  
Securities available-for-sale, at fair value 3,677,019,000 3,586,681,000
Securities held-to-maturity [Abstract]    
Amortized Cost 1,014,543,000 1,028,550,000
Allowance for credit losses - securities held-to-maturity (198,000) (191,000)
HeldToMaturitySecurities, net of allowance for credit losses 1,014,345,000 1,028,359,000
Gross Unrealized Gains 22,320,000 38,272,000
Gross Unrealized Losses 3,114,000 291,000
Securities held-to-maturity, fair value 1,033,749,000 1,066,531,000
Held-to-maturity, Amortized Cost [Abstract]    
Due in one year or less 0  
Due in one year to five years 1,409,000  
Due in five years to ten years 7,152,000  
Due after ten years 1,005,982,000  
Mortgage-backed securities 0  
Asset-backed securities 0  
Amortized Cost 1,014,543,000 1,028,550,000
Held-to-maturity, Fair Value [Abstract]    
Due in one year or less 0  
Due in one year to five years 1,476,000  
Due in five years to ten years 7,223,000  
Due after ten years 1,025,050,000  
Mortgage-backed securities 0  
Asset-backed securities 0  
Fair Value 1,033,749,000 1,066,531,000
U.S. Treasury securities    
Securities available-for-sale [Abstract]    
Amortized Cost 83,971,000 82,199,000
Gross Unrealized Gains 22,000 10,000
Gross Unrealized Losses 0 0
Securities available-for-sale, at fair value 83,993,000 82,209,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 83,993,000 82,209,000
U.S. government agency securities    
Securities available-for-sale [Abstract]    
Amortized Cost 159,613,000 74,916,000
Gross Unrealized Gains 1,224,000 1,547,000
Gross Unrealized Losses 3,381,000 60,000
Securities available-for-sale, at fair value 157,456,000 76,403,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 157,456,000 76,403,000
Mortgage-backed securities    
Securities available-for-sale [Abstract]    
Amortized Cost 1,583,670,000 1,623,759,000
Gross Unrealized Gains 40,921,000 67,759,000
Gross Unrealized Losses 14,813,000 2,327,000
Securities available-for-sale, at fair value 1,609,778,000 1,689,191,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 1,609,778,000 1,689,191,000
State and municipal securities    
Securities available-for-sale [Abstract]    
Amortized Cost 1,480,691,000 1,411,288,000
Gross Unrealized Gains 40,140,000 44,559,000
Gross Unrealized Losses 8,471,000 12,484,000
Securities available-for-sale, at fair value 1,512,360,000 1,443,363,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 1,512,360,000 1,443,363,000
Securities held-to-maturity [Abstract]    
Amortized Cost 1,014,543,000 1,028,550,000
Gross Unrealized Gains 22,320,000 38,272,000
Gross Unrealized Losses 3,114,000 291,000
Securities held-to-maturity, fair value 1,033,749,000 1,066,531,000
Held-to-maturity, Amortized Cost [Abstract]    
Amortized Cost 1,014,543,000 1,028,550,000
Held-to-maturity, Fair Value [Abstract]    
Fair Value 1,033,749,000 1,066,531,000
Asset-backed securities    
Securities available-for-sale [Abstract]    
Amortized Cost 191,005,000 177,878,000
Gross Unrealized Gains 382,000 715,000
Gross Unrealized Losses 692,000 657,000
Securities available-for-sale, at fair value 190,695,000 177,936,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 190,695,000 177,936,000
Corporate notes and other    
Securities available-for-sale [Abstract]    
Amortized Cost 121,766,000 117,256,000
Gross Unrealized Gains 2,824,000 2,632,000
Gross Unrealized Losses 1,853,000 2,309,000
Securities available-for-sale, at fair value 122,737,000 117,579,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value $ 122,737,000 $ 117,579,000