Securities (Amortized Cost and Fair Value of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Debt Securities Available for Sale, Due in one year or less, Amortized Cost	$ 6,598
Debt Securities Available for Sale, Due after one year through five years, Amortized Cost	1,529,328
Debt Securities Available for Sale, Due after five years through ten years, Amortized Cost	1,569,872
Debt Securities Available for Sale, Due after ten years, Amortized Cost	375,151
Total available for sale debt securities, Amortized Cost	3,480,949
Debt Securities Available for Sale, Due in one year or less, Fair Value	6,629
Debt Securities Available for Sale, Due after one year through five years, Fair Value	1,558,458
Debt Securities Available for Sale, Due after five years through ten years, Fair Value	1,606,207
Debt Securities Available for Sale, Due after ten years, Fair Value	378,363
Total available for sale debt securities, Fair Value	3,549,657
Debt Securities Held to Maturity, Due in one year or less, Amortized Cost	74,476
Debt Securities Held to Maturity, Due after one year through five years, Amortized Cost	1,852,708
Debt Securities Held to Maturity, Due after five years through ten years, Amortized Cost	927,878
Total held to maturity debt securities, Amortized Cost	2,855,062	$ 2,979,547
Debt Securities Held to Maturity, Due in one year or less, Fair Value	74,611
Debt Securities Held to Maturity, Due after one year through five years, Fair Value	1,864,562
Debt Securities Held to Maturity, Due after five years through ten years, Fair Value	981,183
Total held to maturity debt securities, Fair Value	$ 2,920,356	$ 2,935,856