Investment Securities - Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	$ 3,368,525	$ 3,252,586
Available-for-Sale, Unrealized Gains	5,827	7,943
Available-for-Sale, Unrealized Losses	(101,896)	(64,322)
Available-for-Sale securities, Fair Value	3,272,456	3,196,207	$ 2,775,837
Held-to-Maturity, Amortized Cost	512,207	684,063	688,951
Held-to-Maturity, Unrealized Gains	5,736	43,648
Held-to-Maturity, Unrealized Losses	(12,377)	(8)
Held-to-Maturity, Fair Value	505,566	727,703	740,564
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	5,322	5,473
Available-for-Sale, Unrealized Gains		83
Available-for-Sale, Unrealized Losses	(85)	(5)
Available-for-Sale securities, Fair Value	5,237	5,551	5,615
U.S. Government-Sponsored Entities and Agencies [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	631,360	675,643
Available-for-Sale, Unrealized Gains		3
Available-for-Sale, Unrealized Losses	(19,664)	(11,360)
Available-for-Sale securities, Fair Value	611,696	664,286	576,436
Held-to-Maturity, Amortized Cost	73,776
Held-to-Maturity, Unrealized Losses	(3,144)
Held-to-Maturity, Fair Value	70,632
Mortgage-Backed Securities - Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	1,571,101	1,704,014
Available-for-Sale, Unrealized Gains	402	1,600
Available-for-Sale, Unrealized Losses	(62,811)	(37,932)
Available-for-Sale securities, Fair Value	1,508,692	1,667,682	1,450,736
Held-to-Maturity, Amortized Cost	131,562	6,903
Held-to-Maturity, Unrealized Gains	45	153
Held-to-Maturity, Unrealized Losses	(4,995)
Held-to-Maturity, Fair Value	126,612	7,056
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	794,671	529,835
Available-for-Sale, Unrealized Gains	5,200	5,085
Available-for-Sale, Unrealized Losses	(7,205)	(4,727)
Available-for-Sale securities, Fair Value	792,666	530,193	$ 414,673
Held-to-Maturity, Amortized Cost	306,869	677,160
Held-to-Maturity, Unrealized Gains	5,691	43,495
Held-to-Maturity, Unrealized Losses	(4,238)	(8)
Held-to-Maturity, Fair Value	308,322	720,647
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	13,873	16,605
Available-for-Sale, Unrealized Losses	(4,801)	(8,157)
Available-for-Sale securities, Fair Value	9,072	8,448
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	352,198	321,016
Available-for-Sale, Unrealized Gains	225	1,172
Available-for-Sale, Unrealized Losses	(7,330)	(2,141)
Available-for-Sale securities, Fair Value	$ 345,093	$ 320,047