Securities (Amortized Cost and Fair Value of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Debt Securities Available for Sale, Due in one year or less, Amortized Cost	$ 16,304
Debt Securities Available for Sale, Due after one year through five years, Amortized Cost	179,035
Debt Securities Available for Sale, Due after five years through ten years, Amortized Cost	1,874,212
Debt Securities Available for Sale, Due after ten years, Amortized Cost	2,628,753
Securities Available for Sale, Gross Amortized Cost	4,698,304	$ 4,637,610
Debt Securities Available for Sale, Due in one year or less, Fair Value	16,436
Debt Securities Available for Sale, Due after one year through five years, Fair Value	191,064
Debt Securities Available for Sale, Due after five years through ten years, Fair Value	2,006,374
Debt Securities Available for Sale, Due after ten years, Fair Value	2,718,268
Total available for sale debt securities, Fair Value	4,932,142	4,675,304
Debt Securities Held to Maturity, Due in one year or less, Amortized Cost	2,199
Debt Securities Held to Maturity, Due after one year through five years, Amortized Cost	189,341
Debt Securities Held to Maturity, Due after five years through ten years, Amortized Cost	622,579
Debt Securities Held to Maturity, Due after ten years, Amortized Cost	635,542
Total held to maturity debt securities, Amortized Cost	1,449,661	1,568,009
Debt Securities Held to Maturity, Due in one year or less, Fair Value	2,191
Debt Securities Held to Maturity, Due after one year through five years, Fair Value	201,759
Debt Securities Held to Maturity, Due after five years through ten years, Fair Value	683,858
Debt Securities Held to Maturity, Due after ten years, Fair Value	665,785
Total held to maturity debt securities, Fair Value	$ 1,553,593	$ 1,611,004