Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 53,698	$ 165,286
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	217	751
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	18,606	19,556
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	222	200
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	72,304	184,842
Held-to-maturity Securities, Gross Unrealized Losses	439	951
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	50,185	148,851
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	725	1,521
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	15,046	15,449
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,300	1,765
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	65,231	164,300
Available-for Sale, Gross Unrealized Losses, Total	2,025	3,286
Fair Value of Investments with Unrealized Losses, Less Than 12 months	103,883	314,137
Gross Unrealized Loss, Less Than 12 Months	942	2,272
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	33,652	35,005
Gross Unrealized Losses, 12 Months or Greater	1,522	1,965
Fair Value of Investments with Unrealized Losses, Total	137,535	349,142
Gross Unrealized Losses, Total	2,464	4,237
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		2,980
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss		4
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value		2,980
Held-to-maturity Securities, Gross Unrealized Losses		4
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	39,485	128,436
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	184	687
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	17,255	18,139
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	131	123
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	56,740	146,575
Held-to-maturity Securities, Gross Unrealized Losses	315	810
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	43,457	141,857
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	688	1,475
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	3,197	4,216
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	35	11
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	46,654	146,073
Available-for Sale, Gross Unrealized Losses, Total	723	1,486
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	11,130	24,068
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	22	45
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	11,130	24,068
Held-to-maturity Securities, Gross Unrealized Losses	22	45
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	4,223	6,560
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	3	9
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	4,637	4,439
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	169	198
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	8,860	10,999
Available-for Sale, Gross Unrealized Losses, Total	172	207
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	33	28
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	1	1
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	33	46
Held-to-maturity Securities, Gross Unrealized Losses	1	1
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	2,505	434
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	34	37
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	7,212	6,794
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,096	1,556
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	9,717	7,228
Available-for Sale, Gross Unrealized Losses, Total	1,130	1,593
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	3,083	9,784
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	11	15
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	3,083	9,784
Held-to-maturity Securities, Gross Unrealized Losses	11	15
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,318	1,389
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	90	76
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	1,318	1,389
Held-to-maturity Securities, Gross Unrealized Losses	$ 90	$ 76