Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 495,548	$ 1,012,122	$ 184,616
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	2,621	15,712	1,004
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	198,307	47,699	71,767
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	3,691	1,047	1,100
Available-for-sale securities, continuous unrealized loss position, Fair Value	693,855	1,059,821	256,383
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	6,312	16,759	2,104
Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	45,050
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	30
Available-for-sale securities, continuous unrealized loss position, Fair Value	45,050
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	30
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	54,983	446,052	82,131
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	309	9,997	711
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	45,217	1,209	741
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	1,285	16	4
Available-for-sale securities, continuous unrealized loss position, Fair Value	100,200	447,261	82,872
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	1,594	10,013	715
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value		244	12,257
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		3	1
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	240
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	2
Available-for-sale securities, continuous unrealized loss position, Fair Value	240	244	12,257
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	2	3	1
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	225,369	372,331	80,015
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1,531	4,532	267
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	131,849	33,227	57,334
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	2,266	981	1,032
Available-for-sale securities, continuous unrealized loss position, Fair Value	357,218	405,558	137,349
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	3,797	5,513	1,299
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	170,146	193,495	10,213
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	751	1,180	25
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	21,001	13,263	13,692
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	138	50	64
Available-for-sale securities, continuous unrealized loss position, Fair Value	191,147	206,758	23,905
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 889	$ 1,230	$ 89