Investment Securities - Gross Unrealized Losses and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Total temporarily impaired investment securities, Estimated Fair Value, Less than 12 Months	$ 181,893	$ 38,804	$ 75,996
Total temporarily impaired investment securities, Unrealized Losses, Less than 12 Months	3,074	100	453
Total temporarily impaired investment securities, Estimated Fair Value, 12 Months or More	14,696	82,040	137,608
Total temporarily impaired investment securities, Unrealized Losses, 12 Months or More	287	1,058	4,231
Total temporarily impaired securities, Estimated Fair Value, Total	196,589	120,844	213,604
Total temporarily impaired securities, Unrealized Losses, Total	3,361	1,158	4,684
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Total temporarily impaired investment securities, Estimated Fair Value, Less than 12 Months	104,621	29,174	60,769
Total temporarily impaired investment securities, Unrealized Losses, Less than 12 Months	1,532	75	386
Total temporarily impaired investment securities, Estimated Fair Value, 12 Months or More	7,515	34,414	79,000
Total temporarily impaired investment securities, Unrealized Losses, 12 Months or More	98	318	2,118
Total temporarily impaired securities, Estimated Fair Value, Total	112,136	63,588	139,769
Total temporarily impaired securities, Unrealized Losses, Total	1,630	393	2,504
U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total temporarily impaired investment securities, Estimated Fair Value, Less than 12 Months	76,252	9,630	15,227
Total temporarily impaired investment securities, Unrealized Losses, Less than 12 Months	1,534	25	67
Total temporarily impaired investment securities, Estimated Fair Value, 12 Months or More	7,181	47,626	58,608
Total temporarily impaired investment securities, Unrealized Losses, 12 Months or More	189	740	2,113
Total temporarily impaired securities, Estimated Fair Value, Total	83,433	57,256	73,835
Total temporarily impaired securities, Unrealized Losses, Total	1,723	$ 765	$ 2,180
CRA Qualified Investment Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Total temporarily impaired investment securities, Estimated Fair Value, Less than 12 Months	1,020
Total temporarily impaired investment securities, Unrealized Losses, Less than 12 Months	8
Total temporarily impaired investment securities, Estimated Fair Value, 12 Months or More	0
Total temporarily impaired investment securities, Unrealized Losses, 12 Months or More	0
Total temporarily impaired securities, Estimated Fair Value, Total	1,020
Total temporarily impaired securities, Unrealized Losses, Total	$ 8