Securities (Amortized Cost and Fair Value of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Securities [Abstract]
Debt Securities Available for Sale, Due in one year or less, Amortized Cost	$ 7,388
Debt Securities Available for Sale, Due after one year through five years, Amortized Cost	45,623
Debt Securities Available for Sale, Due after five years through ten years, Amortized Cost	1,249,127
Debt Securities Available for Sale, Due after ten years, Amortized Cost	1,706,813
Total available for sale debt securities, Amortized Cost	3,008,951
Debt Securities Available for Sale, Due in one year or less, Fair Value	7,408
Debt Securities Available for Sale, Due after one year through five years, Fair Value	45,740
Debt Securities Available for Sale, Due after five years through ten years, Fair Value	1,202,997
Debt Securities Available for Sale, Due after ten years, Fair Value	1,659,503
Total available for sale debt securities, Fair Value	2,915,648
Debt Securities Held to Maturity, Due in one year or less, Amortized Cost	4,190
Debt Securities Held to Maturity, Due after one year through five years, Amortized Cost	132,047
Debt Securities Held to Maturity, Due after five years through ten years, Amortized Cost	1,478,419
Debt Securities Held to Maturity, Due after ten years, Amortized Cost	1,399,772
Total held to maturity debt securities, Amortized Cost	3,014,428	$ 2,977,511
Debt Securities Held to Maturity, Due in one year or less, Fair Value	4,205
Debt Securities Held to Maturity, Due after one year through five years, Fair Value	131,197
Debt Securities Held to Maturity, Due after five years through ten years, Fair Value	1,446,343
Debt Securities Held to Maturity, Due after ten years, Fair Value	1,370,550
Total held to maturity debt securities, Fair Value	$ 2,952,295	$ 2,962,010