Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 999,053	$ 1,012,122	$ 47,509
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	13,394	15,712	393
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	45,561	47,699	63,715
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	966	1,047	669
Available-for-sale securities, continuous unrealized loss position, Fair Value	1,044,614	1,059,821	111,224
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	14,360	16,759	1,062
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	259,419	446,052	6,804
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	7,391	9,997	23
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	1,215	1,209	1,071
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	8	16	5
Available-for-sale securities, continuous unrealized loss position, Fair Value	260,634	447,261	7,875
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	7,399	10,013	28
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	494,836	372,331	25,152
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	4,544	4,532	357
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	31,558	33,227	48,124
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	893	981	606
Available-for-sale securities, continuous unrealized loss position, Fair Value	526,394	405,558	73,276
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	5,437	5,513	963
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	225,789	193,495	15,553
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1,454	1,180	13
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	12,788	13,263	14,520
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	65	50	58
Available-for-sale securities, continuous unrealized loss position, Fair Value	238,577	206,758	30,073
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	1,519	1,230	$ 71
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	9,001	244
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	3	3
Available-for-sale securities, continuous unrealized loss position, Fair Value	9,001	244
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	3	$ 3
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	10,008
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	10,008
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 2