Securities (Amortized Cost and Fair Value of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Securities [Abstract]
Debt Securities Available for Sale, Due in one year or less, Amortized Cost	$ 5,392
Debt Securities Available for Sale, Due after one year through five years, Amortized Cost	46,852
Debt Securities Available for Sale, Due after five years through ten years, Amortized Cost	1,097,850
Debt Securities Available for Sale, Due after ten years, Amortized Cost	1,729,249
Total available for sale debt securities, Amortized Cost	2,879,343
Debt Securities Available for Sale, Due in one year or less, Fair Value	5,425
Debt Securities Available for Sale, Due after one year through five years, Fair Value	47,394
Debt Securities Available for Sale, Due after five years through ten years, Fair Value	1,082,709
Debt Securities Available for Sale, Due after ten years, Fair Value	1,719,750
Total available for sale debt securities, Fair Value	2,855,278
Debt Securities Held to Maturity, Due in one year or less, Amortized Cost	10,081
Debt Securities Held to Maturity, Due after one year through five years, Amortized Cost	108,164
Debt Securities Held to Maturity, Due after five years through ten years, Amortized Cost	837,353
Debt Securities Held to Maturity, Due after ten years, Amortized Cost	1,813,676
Total held to maturity debt securities, Amortized Cost	2,769,274	$ 2,500,220
Debt Securities Held to Maturity, Due in one year or less, Fair Value	10,123
Debt Securities Held to Maturity, Due after one year through five years, Fair Value	108,726
Debt Securities Held to Maturity, Due after five years through ten years, Fair Value	833,465
Debt Securities Held to Maturity, Due after ten years, Fair Value	1,815,834
Total held to maturity debt securities, Fair Value	$ 2,768,148	$ 2,470,117