Securities (Amortized Cost and Fair Value of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Debt Securities Available for Sale, Due in one year or less, Amortized Cost	$ 3,787
Debt Securities Available for Sale, Due after one year through five years, Amortized Cost	259,889
Debt Securities Available for Sale, Due after five years through ten years, Amortized Cost	2,861,768
Debt Securities Available for Sale, Due after ten years, Amortized Cost	3,484,449
Securities Available for Sale, Gross Amortized Cost	6,609,893	$ 5,766,234
Debt Securities Available for Sale, Due in one year or less, Fair Value	3,789
Debt Securities Available for Sale, Due after one year through five years, Fair Value	278,591
Debt Securities Available for Sale, Due after five years through ten years, Fair Value	2,875,550
Debt Securities Available for Sale, Due after ten years, Fair Value	3,472,718
Total available for sale debt securities, Fair Value	6,630,648	5,999,327
Debt Securities Held to Maturity, Due in one year or less, Amortized Cost	2,339
Debt Securities Held to Maturity, Due after one year through five years, Amortized Cost	225,806
Debt Securities Held to Maturity, Due after five years through ten years, Amortized Cost	662,066
Debt Securities Held to Maturity, Due after ten years, Amortized Cost	485,131
Total held to maturity debt securities, Amortized Cost	1,375,342	1,357,170
Debt Securities Held to Maturity, Due in one year or less, Fair Value	2,341
Debt Securities Held to Maturity, Due after one year through five years, Fair Value	239,613
Debt Securities Held to Maturity, Due after five years through ten years, Fair Value	704,949
Debt Securities Held to Maturity, Due after ten years, Fair Value	508,321
Total held to maturity debt securities, Fair Value	$ 1,455,224	$ 1,467,581