Investment Securities (Summary Of Debt Securities By Contractual Maturity) (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale, Amortized Cost, Fiscal Year Maturity [Abstract]
Available-for-sale, Amortized Cost - Due in one year or less	$ 9,997
Available-for-sale, Amortized Cost - Due after one year through five years	17,000
Available-for-sale, Amortized Cost - Due after five through ten years	308,645
Available-for-sale, Amortized Cost - Due after ten years	2,655,008
Available-for-sale, Amortized Cost - Total debt securities	2,990,650	$ 2,994,616
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Available-for-sale, Fair Value - Due in one year or less	9,998
Available-for-sale, Fair Value - Due after one year through five years	17,032
Available-for-sale, Fair Value - Due after five through ten years	308,548
Available-for-sale, Fair Value - Due after ten years	2,643,079
Debt Securities, Available-for-sale	2,978,657	2,898,730
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract]
Held-to-maturity, Amortized Cost - Due in one year or less	2,350
Held-to-maturity, Amortized Cost - Due after one year through five years	6,812
Held-to-maturity, Amortized Cost - Due after five through ten years	112,156
Held-to-maturity, Amortized Cost - Due after ten years	4,515,389
Amortized Cost	4,636,707	4,325,420
Debt Securities, Held-to-maturity, Maturity, Fair Value [Abstract]
Held to maturity, Fair Value - Due in one year or less	2,357
Held-to-maturity, Fair Value - Due after one year through five years	6,959
Held to maturity, Fair Value - Due after five through ten years	115,431
Held to maturity, Fair Value - Due after ten years	4,549,763
Fair Value	$ 4,674,510	$ 4,209,121