Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 169,945	$ 150,147
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	1,319	582
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	12,412	1,662
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	97	82
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	182,357	151,809
Held-to-maturity Securities, Gross Unrealized Losses	1,416	664
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	109,774	110,622
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	673	487
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	16,220	18,104
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	2,386	3,007
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	125,994	128,726
Available-for Sale, Gross Unrealized Losses, Total	3,059	3,494
Fair Value of Investments with Unrealized Losses, Less Than 12 months	279,719	260,769
Gross Unrealized Loss, Less Than 12 Months	1,992	1,069
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	28,632	19,766
Gross Unrealized Losses, 12 Months or Greater	2,483	3,089
Fair Value of Investments with Unrealized Losses, Total	308,351	280,535
Gross Unrealized Losses, Total	4,475	4,158
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		3,385
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss		2
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value		3,385
Held-to-maturity Securities, Gross Unrealized Losses		2
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	92,732	129,788
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	356	520
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	10,852
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	28
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	103,584	129,788
Held-to-maturity Securities, Gross Unrealized Losses	384	520
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	76,180	106,185
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	323	470
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	4,085
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	7
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	80,265	106,185
Available-for Sale, Gross Unrealized Losses, Total	330	470
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	77,142	16,896
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	961	58
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	77,142	16,896
Held-to-maturity Securities, Gross Unrealized Losses	961	58
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	33,017	3,810
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	333	6
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	4,392	10,347
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	276	1,666
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	37,409	14,157
Available-for Sale, Gross Unrealized Losses, Total	609	1,672
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	71	78
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	2	2
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	71	78
Held-to-maturity Securities, Gross Unrealized Losses	2	2
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	577	627
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	17	11
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	7,743	7,757
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	2,103	1,341
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	8,320	8,384
Available-for Sale, Gross Unrealized Losses, Total	2,120	1,352
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,560	1,662
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	69	82
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	1,560	1,662
Held-to-maturity Securities, Gross Unrealized Losses	$ 69	$ 82