Securities (Amortized Cost and Fair Value of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Debt Securities Available for Sale, Due in one year or less, Amortized Cost	$ 2,290
Debt Securities Available for Sale, Due after one year through five years, Amortized Cost	636,212
Debt Securities Available for Sale, Due after five years through ten years, Amortized Cost	2,805,443
Debt Securities Available for Sale, Due after ten years, Amortized Cost	3,499,566
Securities Available for Sale, Gross Amortized Cost	6,943,511	$ 5,766,234
Debt Securities Available for Sale, Due in one year or less, Fair Value	2,294
Debt Securities Available for Sale, Due after one year through five years, Fair Value	670,865
Debt Securities Available for Sale, Due after five years through ten years, Fair Value	2,839,375
Debt Securities Available for Sale, Due after ten years, Fair Value	3,488,387
Total available for sale debt securities, Fair Value	7,000,921	5,999,327
Debt Securities Held to Maturity, Due in one year or less, Amortized Cost	11,226
Debt Securities Held to Maturity, Due after one year through five years, Amortized Cost	336,328
Debt Securities Held to Maturity, Due after five years through ten years, Amortized Cost	548,812
Debt Securities Held to Maturity, Due after ten years, Amortized Cost	411,335
Total held to maturity debt securities, Amortized Cost	1,307,701	1,357,170
Debt Securities Held to Maturity, Due in one year or less, Fair Value	11,371
Debt Securities Held to Maturity, Due after one year through five years, Fair Value	354,897
Debt Securities Held to Maturity, Due after five years through ten years, Fair Value	587,548
Debt Securities Held to Maturity, Due after ten years, Fair Value	432,151
Total held to maturity debt securities, Fair Value	$ 1,385,967	$ 1,467,581