Securities (Amortized Cost and Fair Value of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Securities [Abstract]
Debt Securities Available for Sale, Due in one year or less, Amortized Cost	$ 7,624
Debt Securities Available for Sale, Due after one year through five years, Amortized Cost	40,743
Debt Securities Available for Sale, Due after five years through ten years, Amortized Cost	1,226,635
Debt Securities Available for Sale, Due after ten years, Amortized Cost	1,674,055
Total available for sale debt securities, Amortized Cost	2,949,057
Debt Securities Available for Sale, Due in one year or less, Fair Value	7,658
Debt Securities Available for Sale, Due after one year through five years, Fair Value	41,083
Debt Securities Available for Sale, Due after five years through ten years, Fair Value	1,208,735
Debt Securities Available for Sale, Due after ten years, Fair Value	1,653,393
Total available for sale debt securities, Fair Value	2,910,869
Debt Securities Held to Maturity, Due in one year or less, Amortized Cost	10,686
Debt Securities Held to Maturity, Due after one year through five years, Amortized Cost	122,555
Debt Securities Held to Maturity, Due after five years through ten years, Amortized Cost	1,052,688
Debt Securities Held to Maturity, Due after ten years, Amortized Cost	1,791,582
Total held to maturity debt securities, Amortized Cost	2,977,511	$ 2,500,220
Debt Securities Held to Maturity, Due in one year or less, Fair Value	10,695
Debt Securities Held to Maturity, Due after one year through five years, Fair Value	122,339
Debt Securities Held to Maturity, Due after five years through ten years, Fair Value	1,051,036
Debt Securities Held to Maturity, Due after ten years, Fair Value	1,777,940
Total held to maturity debt securities, Fair Value	$ 2,962,010	$ 2,470,117