Securities (Amortized Cost and Fair Value of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Securities [Abstract]
Debt Securities Available for Sale, Due in one year or less, Amortized Cost	$ 5,073
Debt Securities Available for Sale, Due after one year through five years, Amortized Cost	57,191
Debt Securities Available for Sale, Due after five years through ten years, Amortized Cost	1,279,427
Debt Securities Available for Sale, Due after ten years, Amortized Cost	1,749,929
Total available for sale debt securities, Amortized Cost	3,091,620
Debt Securities Available for Sale, Due in one year or less, Fair Value	5,088
Debt Securities Available for Sale, Due after one year through five years, Fair Value	57,118
Debt Securities Available for Sale, Due after five years through ten years, Fair Value	1,225,729
Debt Securities Available for Sale, Due after ten years, Fair Value	1,693,738
Total available for sale debt securities, Fair Value	2,981,673
Debt Securities Held to Maturity, Due in one year or less, Amortized Cost	17,258
Debt Securities Held to Maturity, Due after one year through five years, Amortized Cost	137,093
Debt Securities Held to Maturity, Due after five years through ten years, Amortized Cost	1,474,871
Debt Securities Held to Maturity, Due after ten years, Amortized Cost	1,502,978
Total held to maturity debt securities, Amortized Cost	3,132,200	$ 2,977,511
Debt Securities Held to Maturity, Due in one year or less, Fair Value	17,413
Debt Securities Held to Maturity, Due after one year through five years, Fair Value	135,198
Debt Securities Held to Maturity, Due after five years through ten years, Fair Value	1,436,090
Debt Securities Held to Maturity, Due after ten years, Fair Value	1,465,768
Total held to maturity debt securities, Fair Value	$ 3,054,469	$ 2,962,010