Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 2,623,934	$ 2,654,885	$ 2,655,242
Securities available-for-sale, Gross Unrealized Holding Gains	107,200	84,855	84,454
Securities available-for-sale, Gross Unrealized Holding Losses	(2,104)	(5,841)	(2,343)
Securities available-for-sale, Estimated Fair Value	2,729,030	2,733,899	2,737,353
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	10,685	10,792	10,828
Securities available-for-sale, Gross Unrealized Holding Gains	54	5	89
Securities available-for-sale, Gross Unrealized Holding Losses		(2)
Securities available-for-sale, Estimated Fair Value	10,739	10,795	10,917
Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	114,918	148,393	153,960
Securities available-for-sale, Gross Unrealized Holding Gains	802	268	1,237
Securities available-for-sale, Gross Unrealized Holding Losses		(107)
Securities available-for-sale, Estimated Fair Value	115,720	148,554	155,197
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,419,737	1,379,879	1,375,958
Securities available-for-sale, Gross Unrealized Holding Gains	83,694	71,382	62,005
Securities available-for-sale, Gross Unrealized Holding Losses	(715)	(134)	(812)
Securities available-for-sale, Estimated Fair Value	1,502,716	1,451,127	1,437,151
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	68,285	86,182	88,927
Securities available-for-sale, Gross Unrealized Holding Gains	1,325	1,778	2,382
Securities available-for-sale, Gross Unrealized Holding Losses	(1)	(5)
Securities available-for-sale, Estimated Fair Value	69,609	87,955	91,309
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	750,673	781,648	817,791
Securities available-for-sale, Gross Unrealized Holding Gains	17,125	10,993	16,566
Securities available-for-sale, Gross Unrealized Holding Losses	(1,299)	(3,759)	(1,498)
Securities available-for-sale, Estimated Fair Value	766,499	788,882	832,859
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	259,636	247,991	207,778
Securities available-for-sale, Gross Unrealized Holding Gains	4,200	429	2,175
Securities available-for-sale, Gross Unrealized Holding Losses	(89)	(1,834)	(33)
Securities available-for-sale, Estimated Fair Value	$ 263,747	$ 246,586	$ 209,920