Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 33,408	$ 165,286
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	152	751
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	4,394	19,556
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	91	200
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	37,802	184,842
Held-to-maturity Securities, Gross Unrealized Losses	243	951
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	33,086	148,851
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	425	1,521
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	19,576	15,449
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,033	1,765
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	52,662	164,300
Available-for Sale, Gross Unrealized Losses, Total	1,458	3,286
Fair Value of Investments with Unrealized Losses, Less Than 12 months	66,494	314,137
Gross Unrealized Loss, Less Than 12 Months	577	2,272
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	23,970	35,005
Gross Unrealized Losses, 12 Months or Greater	1,124	1,965
Fair Value of Investments with Unrealized Losses, Total	90,464	349,142
Gross Unrealized Losses, Total	1,701	4,237
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	23,075	24,068
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	96	45
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	23,075	24,068
Held-to-maturity Securities, Gross Unrealized Losses	96	45
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	22,147	6,560
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	328	9
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	4,746	4,439
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	31	198
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	26,893	10,999
Available-for Sale, Gross Unrealized Losses, Total	359	207
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	9,696	128,436
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	55	687
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	3,173	18,139
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	26	123
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	12,869	146,575
Held-to-maturity Securities, Gross Unrealized Losses	81	810
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	7,673	141,857
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	9	1,475
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	7,900	4,216
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	205	11
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	15,573	146,073
Available-for Sale, Gross Unrealized Losses, Total	214	1,486
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		9,784
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss		15
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value		9,784
Held-to-maturity Securities, Gross Unrealized Losses		15
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		18
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		28
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss		1
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value		46
Held-to-maturity Securities, Gross Unrealized Losses		1
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months		434
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months		37
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	6,930	6,794
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	797	1,556
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	6,930	7,228
Available-for Sale, Gross Unrealized Losses, Total	797	1,593
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,221	1,389
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	65	76
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	1,221	1,389
Held-to-maturity Securities, Gross Unrealized Losses	65	76
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	637	2,980
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	1	4
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	637	2,980
Held-to-maturity Securities, Gross Unrealized Losses	1	$ 4
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	3,266
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	88
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	3,266
Available-for Sale, Gross Unrealized Losses, Total	$ 88