Fair Value Disclosures - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	$ 2,885,483	$ 2,860,837	$ 2,729,030
U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		10,668	10,739
Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	73,588	113,703	115,720
Obligations of States and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,434,194	1,564,276	1,502,716
Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,029,284	851,361	766,499
Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	328,862	268,436	263,747
Fair Value, Measurements, Recurring [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	2,885,483	2,860,837	2,729,030
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	15,099	47,965	65,037
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		10,668	10,739
Fair Value, Measurements, Recurring [Member] | Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	73,588	113,703	115,720
Fair Value, Measurements, Recurring [Member] | Obligations of States and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,434,194	1,564,276	1,502,716
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,029,284	851,361	766,499
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	328,862	268,436	263,747
Fair Value, Measurements, Recurring [Member] | Other Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,456	4,428	4,572
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,456	15,096	15,311
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		10,668	10,739
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member] | Other Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,456	4,428	4,572
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	2,881,027	2,845,741	2,713,719
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	15,099	47,965	65,037
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	73,588	113,703	115,720
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Obligations of States and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,434,194	1,564,276	1,502,716
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,029,284	851,361	766,499
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	$ 328,862	$ 268,436	$ 263,747