Securities (Amortized Cost and Fair Value of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Debt Securities Available for Sale, Due in one year or less, Amortized Cost	$ 2,204
Debt Securities Available for Sale, Due after one year through five years, Amortized Cost	367,341
Debt Securities Available for Sale, Due after five years through ten years, Amortized Cost	3,042,604
Debt Securities Available for Sale, Due after ten years, Amortized Cost	3,778,012
Securities Available for Sale, Gross Amortized Cost	7,190,161	$ 5,766,234
Debt Securities Available for Sale, Due in one year or less, Fair Value	2,210
Debt Securities Available for Sale, Due after one year through five years, Fair Value	390,607
Debt Securities Available for Sale, Due after five years through ten years, Fair Value	3,113,151
Debt Securities Available for Sale, Due after ten years, Fair Value	3,794,453
Total available for sale debt securities, Fair Value	7,300,421	5,999,327
Debt Securities Held to Maturity, Due in one year or less, Amortized Cost	10,341
Debt Securities Held to Maturity, Due after one year through five years, Amortized Cost	243,443
Debt Securities Held to Maturity, Due after five years through ten years, Amortized Cost	644,987
Debt Securities Held to Maturity, Due after ten years, Amortized Cost	433,941
Total held to maturity debt securities, Amortized Cost	1,332,712	1,357,170
Debt Securities Held to Maturity, Due in one year or less, Fair Value	10,505
Debt Securities Held to Maturity, Due after one year through five years, Fair Value	258,189
Debt Securities Held to Maturity, Due after five years through ten years, Fair Value	694,204
Debt Securities Held to Maturity, Due after ten years, Fair Value	457,209
Total held to maturity debt securities, Fair Value	$ 1,420,107	$ 1,467,581