Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 163,377	$ 1,067,714
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	671	5,895
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	1,380,516	377,365
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	28,731	6,608
Available-for-sale securities, continuous unrealized loss position, Fair Value	1,543,893	1,445,079
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	29,402	12,503
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		60,329
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		186
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	301
Available-for-sale securities, continuous unrealized loss position, Fair Value	301	60,329
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss		186
Obligations of State 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	27,489	66,361
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	107	219
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	114,461	44,938
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	1,863	717
Available-for-sale securities, continuous unrealized loss position, Fair Value	141,950	111,299
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	1,970	936
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	4,348	224
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	68	2
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	450	237
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	9	3
Available-for-sale securities, continuous unrealized loss position, Fair Value	4,798	461
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	77	5
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	119,584	701,252
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	483	3,988
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	922,289	239,641
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	21,128	4,928
Available-for-sale securities, continuous unrealized loss position, Fair Value	1,041,873	940,893
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	21,611	8,916
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	1,994	239,548
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	5	1,500
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	343,015	92,549
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	5,731	960
Available-for-sale securities, continuous unrealized loss position, Fair Value	345,009	332,097
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	5,736	$ 2,460
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	9,962
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	8
Available-for-sale securities, continuous unrealized loss position, Fair Value	9,962
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 8