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Securities (Year End Securities with Unrealized Losses Segregated by Length of Impairment) (Detail) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Less than 12 Months, Estimated Fair Value $ 890,533 $ 205,686
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 10,257 541
Held to Maturity, More than 12 Months, Estimated Fair Value 408 7,986
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 4 96
Held to Maturity, Total Estimated Fair Value 890,941 213,672
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss 10,261 637
Available for Sale, Less than 12 Months, Estimated Fair Value 972,926 1,289,004
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 13,056 7,231
Available for Sale, More than 12 Months, Estimated Fair Value 231,436 5,411,669
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 521 101,506
Available for Sale, Total Estimated Fair Value 1,204,362 6,700,673
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 13,577 108,737
US Treasury Securities [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Available for Sale, Less than 12 Months, Estimated Fair Value 636,999 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 12,070 0
Available for Sale, More than 12 Months, Estimated Fair Value 199,980 3,139,639
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 14 29,500
Available for Sale, Total Estimated Fair Value 836,979 3,139,639
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 12,084 29,500
Residential Mortgage Backed Securities [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Less than 12 Months, Estimated Fair Value 519,099 0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 9,361 0
Held to Maturity, More than 12 Months, Estimated Fair Value 408 2,034
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 4 85
Held to Maturity, Total Estimated Fair Value 519,507 2,034
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss 9,365 85
Available for Sale, Less than 12 Months, Estimated Fair Value 276,249 152,682
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 782 205
Available for Sale, More than 12 Months, Estimated Fair Value 31,456 213,982
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 507 6,342
Available for Sale, Total Estimated Fair Value 307,705 366,664
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 1,289 6,547
States and Political Subdivisions [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Less than 12 Months, Estimated Fair Value 371,434 205,686
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 896 541
Held to Maturity, More than 12 Months, Estimated Fair Value 0 5,952
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0 11
Held to Maturity, Total Estimated Fair Value 371,434 211,638
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss 896 552
Available for Sale, Less than 12 Months, Estimated Fair Value 59,678 1,136,322
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 204 7,026
Available for Sale, More than 12 Months, Estimated Fair Value 0 2,058,048
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0 65,664
Available for Sale, Total Estimated Fair Value 59,678 3,194,370
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 204 72,690
Other Investments [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss 0 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax $ 0 $ 0