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Securities (Year End Securities with Unrealized Losses Segregated by Length of Impairment) (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Less than 12 Months, Estimated Fair Value $ 205,686 $ 31,285
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 541 96
Held to Maturity, More than 12 Months, Estimated Fair Value 7,986 74,113
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 96 2,688
Held to Maturity, Total Estimated Fair Value 213,672 105,398
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss 637 2,784
Available for Sale, Less than 12 Months, Estimated Fair Value 1,289,004 2,628,920
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 7,231 12,004
Available for Sale, More than 12 Months, Estimated Fair Value 5,411,669 1,401,340
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 101,506 22,773
Available for Sale, Total Estimated Fair Value 6,700,673 4,030,260
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 108,737 34,777
US Treasury Securities [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Available for Sale, Less than 12 Months, Estimated Fair Value 0 2,336,081
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 9,861
Available for Sale, More than 12 Months, Estimated Fair Value 3,139,639 517,575
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 29,500 5,871
Available for Sale, Total Estimated Fair Value 3,139,639 2,853,656
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 29,500 15,732
Residential Mortgage Backed Securities [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Less than 12 Months, Estimated Fair Value 0 2,694
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 38
Held to Maturity, More than 12 Months, Estimated Fair Value 2,034 0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 85 0
Held to Maturity, Total Estimated Fair Value 2,034 2,694
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss 85 38
Available for Sale, Less than 12 Months, Estimated Fair Value 152,682 144,264
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 205 949
Available for Sale, More than 12 Months, Estimated Fair Value 213,982 45,436
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 6,342 1,301
Available for Sale, Total Estimated Fair Value 366,664 189,700
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 6,547 2,250
States and Political Subdivisions [Member]    
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Less than 12 Months, Estimated Fair Value 205,686 28,591
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 541 58
Held to Maturity, More than 12 Months, Estimated Fair Value 5,952 74,113
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 11 2,688
Held to Maturity, Total Estimated Fair Value 211,638 102,704
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss 552 2,746
Available for Sale, Less than 12 Months, Estimated Fair Value 1,136,322 148,575
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 7,026 1,194
Available for Sale, More than 12 Months, Estimated Fair Value 2,058,048 838,329
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 65,664 15,601
Available for Sale, Total Estimated Fair Value 3,194,370 986,904
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 72,690 16,795
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