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Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Debt Securities Held to Maturity (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 2,391,026 $ 2,173,011
Gross Unrealized Gains 47,947 62,326
Gross Unrealized Losses (25,612) (7,725)
Fair Value 2,413,361 2,227,612
U.S. Treasury securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 68,031 68,126
Gross Unrealized Gains 6,148 7,358
Gross Unrealized Losses 0 0
Fair Value 74,179 75,484
U.S. government agency securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 5,659 6,222
Gross Unrealized Gains 136 291
Gross Unrealized Losses 0 0
Fair Value 5,795 6,513
Obligations of states and state agencies    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 254,991 262,762
Gross Unrealized Gains 6,390 8,060
Gross Unrealized Losses (141) (105)
Fair Value 261,240 270,717
Municipal bonds    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 199,030 207,497
Gross Unrealized Gains 5,406 6,292
Gross Unrealized Losses (2) 0
Fair Value 204,434 213,789
Obligations of states and political subdivisions    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 454,021 470,259
Gross Unrealized Gains 11,796 14,352
Gross Unrealized Losses (143) (105)
Fair Value 465,674 484,506
Residential mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,787,211 1,550,306
Gross Unrealized Gains 29,249 39,603
Gross Unrealized Losses (18,719) (254)
Fair Value 1,797,741 1,589,655
Trust preferred securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 37,354 37,348
Gross Unrealized Gains 52 50
Gross Unrealized Losses (6,678) (7,365)
Fair Value 30,728 30,033
Corporate and other debt securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 38,750 40,750
Gross Unrealized Gains 566 672
Gross Unrealized Losses (72) (1)
Fair Value $ 39,244 $ 41,421