Investment Securities (Summary Of Debt Securities By Contractual Maturity) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Available-for-sale, Amortized Cost - Due in one year or less	$ 18,668
Available-for-sale, Amortized Cost - Due after one year through five years	40,246
Available-for-sale, Amortized Cost - Due after five through ten years	381,547
Available-for-sale, Amortized Cost - Due after ten years	2,172,444
Available-for-sale, Amortized Cost - Total debt securities	2,612,905
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Available-for-sale, Fair Value - Due in one year or less	18,714
Available-for-sale, Fair Value - Due after one year through five years	40,748
Available-for-sale, Fair Value - Due after five through ten years	383,721
Available-for-sale, Fair Value - Due after ten years	2,147,979
Available-for-sale, Fair Value - Total debt securities	2,591,162
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Held-to-maturity, Amortized Cost - Due in one year or less	40,146
Held-to-maturity, Amortized Cost - Due after one year through five years	13,410
Held-to-maturity, Amortized Cost - Due after five through ten years	38,323
Held-to-maturity, Amortized Cost - Due after ten years	4,405,432
Amortized Cost	4,497,311	$ 4,160,658
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Held to maturity, Fair Value - Due in one year or less	40,753
Held-to-maturity, Fair Value - Due after one year through five years	13,684
Held to maturity, Fair Value - Due after five through ten years	39,102
Held to maturity, Fair Value - Due after ten years	4,388,136
Fair Value	$ 4,481,675	$ 4,125,125