Investment Securities (Summary Of Debt Securities By Contractual Maturity) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis	$ 1,247
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis	40,066
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis	332,558
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis	2,301,260
Available-for-sale Debt Securities, Amortized Cost Basis	2,675,131
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	1,247
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	40,447
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	333,931
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	2,262,412
Available for Sale, Fair Value - Total debt securities	2,638,037
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	33,654
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	3,839
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	37,870
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	4,412,029
Amortized Cost	4,487,392	$ 4,160,658
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	34,145
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	3,857
Held-to-maturity Securities, Debt Maturities, Year Six Through Ten, Fair Value	38,450
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	4,379,898
Held-to-maturity Securities, Fair Value	$ 4,456,350	$ 4,125,125