Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 17,594	$ 163,377	$ 1,594,398
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	77	671	30,514
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	549,604	1,380,516	353,912
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	3,544	28,731	11,893
Available-for-sale securities, continuous unrealized loss position, Fair Value	567,198	1,543,893	1,948,310
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	3,621	29,402	42,407
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			303
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss			1
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	303
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss			1
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	136,950
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		107	1,122
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	9,777	114,461	43,239
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	61	1,863	1,574
Available-for-sale securities, continuous unrealized loss position, Fair Value	9,777	141,950	180,189
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	61	1,970	2,696
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	4,617
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		68	76
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value		450	233
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	4,850
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss		77	79
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	17,594	119,584	1,073,327
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	77	483	21,838
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	404,946	922,289	227,155
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	3,024	21,128	8,880
Available-for-sale securities, continuous unrealized loss position, Fair Value	422,540	1,041,873	1,300,482
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	3,101	21,611	30,718
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	379,201
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		5	7,477
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	134,881	343,015	83,285
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	459	5,731	1,436
Available-for-sale securities, continuous unrealized loss position, Fair Value	134,881	345,009	462,486
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 459	5,736	$ 8,913
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