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Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Jun. 30, 2017 | Dec. 31, 2016
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Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | $ 149,638 | $ 206,129
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 838 | 3,283
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 10,637 | 7,465
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 205 | 122
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 160,275 | 213,594
Held-to-maturity securities, Gross Unrealized Losses, Total | 1,043 | 3,405
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 116,577 | 173,573
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 1,768 | 4,113
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 20,307 | 16,052
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 1,405 | 1,454
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 136,884 | 189,625
Available-for Sale, Gross Unrealized Losses, Total | 3,173 | 5,567
Fair Value of Investments with Unrealized Losses, Less Than 12 months | 266,215 | 379,702
Gross Unrealized Loss, Less Than 12 Months | 2,606 | 7,396
Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 30,944 | 23,517
Gross Unrealized Losses, 12 Months or Greater | 1,610 | 1,576
Fair Value of Investments with Unrealized Losses, Total | 297,159 | 403,219
Gross Unrealized Losses, Total | 4,216 | 8,972
States and Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 79,641 | 157,507
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 622 | 3,014
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 3,296 |
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 155 |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 82,937 | 157,507
Held-to-maturity securities, Gross Unrealized Losses, Total | 777 | 3,014
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 33,677 | 73,505
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 1,036 | 2,976
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 8,798 | 4,523
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 420 | 239
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 42,475 | 78,028
Available-for Sale, Gross Unrealized Losses, Total | 1,456 | 3,215
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 46,659 | 44,654
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 142 | 202
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 6,242 | 6,292
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 34 | 63
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 52,901 | 50,946
Held-to-maturity securities, Gross Unrealized Losses, Total | 176 | 265
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 22,167 | 41,419
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 79 | 111
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 7,919 | 7,922
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 118 | 160
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 30,086 | 49,341
Available-for Sale, Gross Unrealized Losses, Total | 197 | 271
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 658 |
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 2 |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 658 |
Held-to-maturity securities, Gross Unrealized Losses, Total | 2 |
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 5,510 | 3,702
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 46 | 48
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 3,590 | 3,607
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater | 867 | 1,055
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 9,100 | 7,309
Available-for Sale, Gross Unrealized Losses, Total | 913 | 1,103
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater | 1,099 | 1,173
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater | 16 | 59
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 1,099 | 1,173
Held-to-maturity securities, Gross Unrealized Losses, Total | 16 | 59
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] |  |
Investment [Line Items] |  |
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months | 22,680 | 3,968
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months | 72 | 67
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total | 22,680 | 3,968
Held-to-maturity securities, Gross Unrealized Losses, Total | 72 | 67
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months | 55,223 | 54,947
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months | 607 | 978
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total | 55,223 | 54,947
Available-for Sale, Gross Unrealized Losses, Total | $ 607 | $ 978

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