Securities (Amortized Cost and Fair Value of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Debt Securities Available for Sale, Due in one year or less, Amortized Cost	$ 14,698
Debt Securities Available for Sale, Due after one year through five years, Amortized Cost	135,349
Debt Securities Available for Sale, Due after five years through ten years, Amortized Cost	1,765,040
Debt Securities Available for Sale, Due after ten years, Amortized Cost	2,793,988
Total available for sale debt securities, Amortized Cost	4,709,075
Debt Securities Available for Sale, Due in one year or less, Fair Value	14,850
Debt Securities Available for Sale, Due after one year through five years, Fair Value	141,030
Debt Securities Available for Sale, Due after five years through ten years, Fair Value	1,865,926
Debt Securities Available for Sale, Due after ten years, Fair Value	2,872,190
Total available for sale debt securities, Fair Value	4,893,996
Debt Securities Held to Maturity, Due after one year through five years, Amortized Cost	136,494
Debt Securities Held to Maturity, Due after five years through ten years, Amortized Cost	668,437
Debt Securities Held to Maturity, Due after ten years, Amortized Cost	675,563
Total held to maturity debt securities, Amortized Cost	1,480,494	$ 1,568,009
Debt Securities Held to Maturity, Due after one year through five years, Fair Value	141,840
Debt Securities Held to Maturity, Due after five years through ten years, Fair Value	718,740
Debt Securities Held to Maturity, Due after ten years, Fair Value	701,997
Total held to maturity debt securities, Fair Value	$ 1,562,577	$ 1,611,004