Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	$ 155,588	$ 618,575
Less than 12 months, Unrealized Losses	(257)	(2,750)
12 months or more, Fair Value	3,892,869	4,465,859
12 months or more, Unrealized Losses	(77,463)	(138,520)
Total Fair Value	4,048,457	5,084,434
Total Unrealized Losses	(77,720)	(141,270)
U.S. Treasury [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value		18,775
Less than 12 months, Unrealized Losses		(4)
12 months or more, Fair Value	38,957	38,552
12 months or more, Unrealized Losses	(1,149)	(1,552)
Total Fair Value	38,957	57,327
Total Unrealized Losses	(1,149)	(1,556)
U.S. Agencies [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	1,936
Less than 12 months, Unrealized Losses	(5)
12 months or more, Fair Value	199	199
12 months or more, Unrealized Losses	(1)	(1)
Total Fair Value	2,135	199
Total Unrealized Losses	(6)	(1)
Mortgage-backed [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	69,379	228,406
Less than 12 months, Unrealized Losses	(165)	(1,256)
12 months or more, Fair Value	2,889,707	3,007,233
12 months or more, Unrealized Losses	(66,974)	(106,967)
Total Fair Value	2,959,086	3,235,639
Total Unrealized Losses	(67,139)	(108,223)
State and political subdivisions [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	60,689	371,394
Less than 12 months, Unrealized Losses	(53)	(1,490)
12 months or more, Fair Value	964,006	1,419,875
12 months or more, Unrealized Losses	(9,339)	(30,000)
Total Fair Value	1,024,695	1,791,269
Total Unrealized Losses	(9,392)	$ (31,490)
Corporates [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	23,584
Less than 12 months, Unrealized Losses	(34)
Total Fair Value	23,584
Total Unrealized Losses	$ (34)