Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position 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 securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 184,616	$ 523,075	$ 166,313
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1,004	3,758	1,197
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	71,767	74,777	77,313
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	1,100	2,083	1,146
Available-for-sale securities, continuous unrealized loss position, Fair Value	256,383	597,852	243,626
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	2,104	5,841	2,343
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	82,131	32,929	97,670
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	711	127	792
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	741	1,513	2,233
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	4	7	20
Available-for-sale securities, continuous unrealized loss position, Fair Value	82,872	34,442	99,903
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	715	134	812
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	80,015	231,481	62,765
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	267	1,765	396
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	57,334	63,919	65,614
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	1,032	1,994	1,102
Available-for-sale securities, continuous unrealized loss position, Fair Value	137,349	295,400	128,379
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	1,299	3,759	1,498
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	10,213	196,163	5,878
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	25	1,752	9
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	13,692	9,345	9,466
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	64	82	24
Available-for-sale securities, continuous unrealized loss position, Fair Value	23,905	205,508	15,344
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	89	1,834	$ 33
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value		5,110
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		2
Available-for-sale securities, continuous unrealized loss position, Fair Value		5,110
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss		2
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		50,388
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		107
Available-for-sale securities, continuous unrealized loss position, Fair Value		50,388
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss		107
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	12,257	7,004
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1	5
Available-for-sale securities, continuous unrealized loss position, Fair Value	12,257	7,004
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 1	$ 5