Investment Securities (Summary Of Debt Securities By Contractual Maturity) (Detail) - USD ($) $ in Thousands	Sep. 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	$ 19,918
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	305,691
Available-for-sale, Amortized Cost - Due after ten years	2,596,583
Available-for-sale, Amortized Cost - Total debt securities	2,939,192	$ 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	19,924
Available-for-sale, Fair Value - Due after one year through five years	17,000
Available-for-sale, Fair Value - Due after five through ten years	305,533
Available-for-sale, Fair Value - Due after ten years	2,617,646
Debt Securities, Available-for-sale	2,960,103	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	1,045
Held-to-maturity, Amortized Cost - Due after one year through five years	6,260
Held-to-maturity, Amortized Cost - Due after five through ten years	222,076
Held-to-maturity, Amortized Cost - Due after ten years	4,964,140
Amortized Cost	5,193,521	4,325,420
Debt Securities, Held-to-maturity, Maturity, Fair Value [Abstract]
Held to maturity, Fair Value - Due in one year or less	1,049
Held-to-maturity, Fair Value - Due after one year through five years	6,392
Held to maturity, Fair Value - Due after five through ten years	228,011
Held to maturity, Fair Value - Due after ten years	5,050,253
Fair Value	$ 5,285,705	$ 4,209,121