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Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Mar. 31, 2017 | Dec. 31, 2016
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Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | $ 159,384 | $ 206,129
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss | 2,156 | 3,283
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 7,377 | 7,465
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss | 107 | 122
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 166,761 | 213,594
Held-to-maturity Securities, Gross Unrealized Losses | 2,263 | 3,405
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 148,413 | 173,573
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 3,522 | 4,113
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 15,963 | 16,052
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 1,182 | 1,454
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 164,376 | 189,625
Available-for Sale, Gross Unrealized Losses, Total | 4,704 | 5,567
Fair Value of Investments with Unrealized Losses, Less Than 12 months | 307,797 | 379,702
Gross Unrealized Loss, Less Than 12 Months | 5,678 | 7,396
Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 23,340 | 23,517
Gross Unrealized Losses, 12 Months or Greater | 1,289 | 1,576
Fair Value of Investments with Unrealized Losses, Total | 331,137 | 403,219
Gross Unrealized Losses, Total | 6,967 | 8,972
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 106,993 | 157,507
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss | 1,895 | 3,014
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 106,993 | 157,507
Held-to-maturity Securities, Gross Unrealized Losses | 1,895 | 3,014
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 60,871 | 73,505
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 2,595 | 2,976
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 4,374 | 4,523
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 199 | 239
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 65,245 | 78,028
Available-for Sale, Gross Unrealized Losses, Total | 2,794 | 3,215
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 46,021 | 44,654
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss | 199 | 202
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 6,258 | 6,292
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss | 57 | 63
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 52,279 | 50,946
Held-to-maturity Securities, Gross Unrealized Losses | 256 | 265
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 28,915 | 41,419
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 58 | 111
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 7,952 | 7,922
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 107 | 160
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 36,867 | 49,341
Available-for Sale, Gross Unrealized Losses, Total | 165 | 271
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 1,119 | 1,173
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss | 50 | 59
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 1,119 | 1,173
Held-to-maturity Securities, Gross Unrealized Losses | 50 | 59
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 3,583 | 3,702
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 35 | 48
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 3,637 | 3,607
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 876 | 1,055
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 7,220 | 7,309
Available-for Sale, Gross Unrealized Losses, Total | 911 | 1,103
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |
Investment [Line Items] |  |
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 6,370 | 3,968
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss | 62 | 67
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | 6,370 | 3,968
Held-to-maturity Securities, Gross Unrealized Losses | 62 | 67
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 55,044 | 54,947
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 834 | 978
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 55,044 | 54,947
Available-for Sale, Gross Unrealized Losses, Total | $ 834 | $ 978

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