Fair Value Disclosures - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	$ 3,276,193	$ 3,087,473	$ 3,018,279
U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value			10,619
Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	50,215	60,330	79,905
Obligations of States and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,320,133	1,420,850	1,564,513
Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,427,772	1,219,097	991,553
Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	468,746	375,737	323,758
Fair Value, Measurements, Recurring [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	3,276,193	3,087,473	3,018,279
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,971	7,031	43,503
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value			10,619
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	50,215	60,330	79,905
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	1,320,133	1,420,850	1,564,513
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,427,772	1,219,097	991,553
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	468,746	375,737	323,758
Fair Value, Measurements, Recurring [Member] | Other Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,356	4,428	4,428
Fair Value, Measurements, Recurring [Member] | Level 1 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,356	4,428	15,047
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			10,619
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	4,356	4,428	4,428
Fair Value, Measurements, Recurring [Member] | Level 2 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	3,271,837	3,083,045	3,003,232
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	4,971	7,031	43,503
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	50,215	60,330	79,905
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	1,320,133	1,420,850	1,564,513
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	1,427,772	1,219,097	991,553
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	$ 468,746	$ 375,737	$ 323,758