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Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($)
Jun. 30, 2021
Dec. 31, 2020
Securities available-for-sale [Abstract]    
Amortized Cost $ 4,238,366,000 $ 3,487,296,000
Gross Unrealized Gains 106,705,000 117,222,000
Gross Unrealized Losses 14,001,000 17,837,000
Securities available-for-sale, at fair value 4,331,070,000 3,586,681,000
Available-for-sale, Amortized Cost [Abstract]    
Due in one year or less 112,691,000  
Due in one year to five years 33,685,000  
Due in five years to ten years 496,477,000  
Due after ten years 1,540,794,000  
Mortgage-backed securities 1,854,519,000  
Asset-backed securities 200,200,000  
Amortized Cost 4,238,366,000  
Available-for-sale, Fair Value [Abstract]    
Due in one year or less 112,686,000  
Due in one year to five years 33,706,000  
Due in five years to ten years 508,583,000  
Due after ten years 1,576,759,000  
Mortgage-backed securities 1,899,248,000  
Asset-backed securities 200,088,000  
Securities available-for-sale, at fair value 4,331,070,000 3,586,681,000
Securities held-to-maturity [Abstract]    
Amortized Cost 996,036,000 1,028,550,000
Allowance for credit losses - securities held-to-maturity (198,000) (191,000)
HeldToMaturitySecurities, net of allowance for credit losses 995,838,000 1,028,359,000
Gross Unrealized Gains 35,035,000 38,272,000
Gross Unrealized Losses 432,000 291,000
Securities held-to-maturity, fair value 1,030,639,000 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 6,609,000  
Due after ten years 988,018,000  
Mortgage-backed securities 0  
Asset-backed securities 0  
Amortized Cost 996,036,000 1,028,550,000
Held-to-maturity, Fair Value [Abstract]    
Due in one year or less 0  
Due in one year to five years 1,470,000  
Due in five years to ten years 6,704,000  
Due after ten years 1,022,465,000  
Mortgage-backed securities 0  
Asset-backed securities 0  
Fair Value 1,030,639,000 1,066,531,000
U.S. Treasury securities    
Securities available-for-sale [Abstract]    
Amortized Cost 111,475,000 82,199,000
Gross Unrealized Gains 5,000 10,000
Gross Unrealized Losses 11,000 0
Securities available-for-sale, at fair value 111,469,000 82,209,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 111,469,000 82,209,000
U.S. government agency securities    
Securities available-for-sale [Abstract]    
Amortized Cost 421,833,000 74,916,000
Gross Unrealized Gains 3,214,000 1,547,000
Gross Unrealized Losses 2,191,000 60,000
Securities available-for-sale, at fair value 422,856,000 76,403,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 422,856,000 76,403,000
Mortgage-backed securities    
Securities available-for-sale [Abstract]    
Amortized Cost 1,854,519,000 1,623,759,000
Gross Unrealized Gains 50,747,000 67,759,000
Gross Unrealized Losses 6,018,000 2,327,000
Securities available-for-sale, at fair value 1,899,248,000 1,689,191,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 1,899,248,000 1,689,191,000
State and municipal securities    
Securities available-for-sale [Abstract]    
Amortized Cost 1,535,461,000 1,411,288,000
Gross Unrealized Gains 48,999,000 44,559,000
Gross Unrealized Losses 3,747,000 12,484,000
Securities available-for-sale, at fair value 1,580,713,000 1,443,363,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 1,580,713,000 1,443,363,000
Securities held-to-maturity [Abstract]    
Amortized Cost 996,036,000 1,028,550,000
Gross Unrealized Gains 35,035,000 38,272,000
Gross Unrealized Losses 432,000 291,000
Securities held-to-maturity, fair value 1,030,639,000 1,066,531,000
Held-to-maturity, Amortized Cost [Abstract]    
Amortized Cost 996,036,000 1,028,550,000
Held-to-maturity, Fair Value [Abstract]    
Fair Value 1,030,639,000 1,066,531,000
Asset-backed securities    
Securities available-for-sale [Abstract]    
Amortized Cost 200,200,000 177,878,000
Gross Unrealized Gains 318,000 715,000
Gross Unrealized Losses 430,000 657,000
Securities available-for-sale, at fair value 200,088,000 177,936,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value 200,088,000 177,936,000
Corporate notes and other    
Securities available-for-sale [Abstract]    
Amortized Cost 114,878,000 117,256,000
Gross Unrealized Gains 3,422,000 2,632,000
Gross Unrealized Losses 1,604,000 2,309,000
Securities available-for-sale, at fair value 116,696,000 117,579,000
Available-for-sale, Fair Value [Abstract]    
Securities available-for-sale, at fair value $ 116,696,000 $ 117,579,000