Investment Securities (Summary Of Debt Securities By Contractual Maturity) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Amortized Cost, Fiscal Year Maturity [Abstract]
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	$ 0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	1,448
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	252,601
Debt Securities, Available-for-sale, Allocated and Single Maturity Date, Maturity, after 10 Years, Amortized Cost	2,980,201
Debt Securities, Available-for-sale, Amortized Cost	3,234,250	$ 2,901,415
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	1,489
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	248,958
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	3,076,329
Available for Sale, Fair Value - Total debt securities	3,326,776	2,925,833
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract]
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	680
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	4,458
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	278,974
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	5,284,076
Amortized Cost	5,568,188	5,293,918
Debt Securities, Held-to-maturity, Maturity, Fair Value [Abstract]
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	685
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	4,711
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	294,528
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	5,535,440
Debt Securities, Held-to-maturity, Fair Value	$ 5,835,364	$ 5,380,653