Investment Securities - Available-for-Sale and Held-to-Maturity Investment Securities with Unrealized Losses by Aggregated Major Security Type and Length of Time in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	$ 1,433,771	$ 1,250,341
Available-for-Sale, Less than 12 Months, Unrealized Losses	(31,626)	(12,358)
Available-for-Sale, 12 months or longer, Fair Value	1,386,673	1,407,920
Available-for-Sale, 12 months or longer, Unrealized Losses	(70,270)	(51,964)
Available-for-Sale, Fair Value	2,820,444	2,658,261
Available-for-Sale, Unrealized Losses	(101,896)	(64,322)
Held-to-Maturity, Less than 12 Months, Fair Value	64,865	2,309
Held-to-Maturity, Less than 12 Months, Unrecognized Losses	(3,419)	(8)
Held-to-Maturity, 12 months or longer, Fair Value	234,010
Held-to-Maturity, 12 months or longer, Unrecognized Losses	(20,574)
Held-to-Maturity, Fair Value	298,875	2,309
Held-to-Maturity, Unrecognized Losses	(23,993)	(8)
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	5,237	1,480
Available-for-Sale, Less than 12 Months, Unrealized Losses	(85)	(5)
Available-for-Sale, Fair Value	5,237	1,480
Available-for-Sale, Unrealized Losses	(85)	(5)
U.S. Government-Sponsored Entities and Agencies [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	202,437	201,773
Available-for-Sale, Less than 12 Months, Unrealized Losses	(5,075)	(1,333)
Available-for-Sale, 12 months or longer, Fair Value	407,060	408,493
Available-for-Sale, 12 months or longer, Unrealized Losses	(14,589)	(10,027)
Available-for-Sale, Fair Value	609,497	610,266
Available-for-Sale, Unrealized Losses	(19,664)	(11,360)
Held-to-Maturity, 12 months or longer, Fair Value	70,632
Held-to-Maturity, 12 months or longer, Unrecognized Losses	(6,368)
Held-to-Maturity, Fair Value	70,632
Held-to-Maturity, Unrecognized Losses	(6,368)
Mortgage-Backed Securities - Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	606,669	789,804
Available-for-Sale, Less than 12 Months, Unrealized Losses	(15,981)	(8,692)
Available-for-Sale, 12 months or longer, Fair Value	845,276	774,825
Available-for-Sale, 12 months or longer, Unrealized Losses	(46,830)	(29,240)
Available-for-Sale, Fair Value	1,451,945	1,564,629
Available-for-Sale, Unrealized Losses	(62,811)	(37,932)
Held-to-Maturity, Less than 12 Months, Fair Value	32,646
Held-to-Maturity, Less than 12 Months, Unrecognized Losses	(1,227)
Held-to-Maturity, 12 months or longer, Fair Value	92,986
Held-to-Maturity, 12 months or longer, Unrecognized Losses	(9,389)
Held-to-Maturity, Fair Value	125,632
Held-to-Maturity, Unrecognized Losses	(10,616)
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	423,498	196,024
Available-for-Sale, Less than 12 Months, Unrealized Losses	(6,703)	(1,899)
Available-for-Sale, 12 months or longer, Fair Value	12,825	90,637
Available-for-Sale, 12 months or longer, Unrealized Losses	(502)	(2,828)
Available-for-Sale, Fair Value	436,323	286,661
Available-for-Sale, Unrealized Losses	(7,205)	(4,727)
Held-to-Maturity, Less than 12 Months, Fair Value	32,219	2,309
Held-to-Maturity, Less than 12 Months, Unrecognized Losses	(2,192)	(8)
Held-to-Maturity, 12 months or longer, Fair Value	70,392
Held-to-Maturity, 12 months or longer, Unrecognized Losses	(4,817)
Held-to-Maturity, Fair Value	102,611	2,309
Held-to-Maturity, Unrecognized Losses	(7,009)	(8)
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, 12 months or longer, Fair Value	9,072	8,448
Available-for-Sale, 12 months or longer, Unrealized Losses	(4,801)	(8,157)
Available-for-Sale, Fair Value	9,072	8,448
Available-for-Sale, Unrealized Losses	(4,801)	(8,157)
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	195,930	61,260
Available-for-Sale, Less than 12 Months, Unrealized Losses	(3,782)	(429)
Available-for-Sale, 12 months or longer, Fair Value	112,440	125,517
Available-for-Sale, 12 months or longer, Unrealized Losses	(3,548)	(1,712)
Available-for-Sale, Fair Value	308,370	186,777
Available-for-Sale, Unrealized Losses	$ (7,330)	$ (2,141)