Securities (Amortized Cost and Fair Value of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Securities [Abstract]
Debt Securities Available for Sale, Due in one year or less, Amortized Cost	$ 4,814
Debt Securities Available for Sale, Due after one year through five years, Amortized Cost	54,339
Debt Securities Available for Sale, Due after five years through ten years, Amortized Cost	1,259,696
Debt Securities Available for Sale, Due after ten years, Amortized Cost	1,730,002
Total available for sale debt securities, Amortized Cost	3,048,851
Debt Securities Available for Sale, Due in one year or less, Fair Value	4,820
Debt Securities Available for Sale, Due after one year through five years, Fair Value	54,077
Debt Securities Available for Sale, Due after five years through ten years, Fair Value	1,196,235
Debt Securities Available for Sale, Due after ten years, Fair Value	1,663,053
Total available for sale debt securities, Fair Value	2,918,185
Debt Securities Held to Maturity, Due in one year or less, Amortized Cost	24,832
Debt Securities Held to Maturity, Due after one year through five years, Amortized Cost	130,348
Debt Securities Held to Maturity, Due after five years through ten years, Amortized Cost	1,452,690
Debt Securities Held to Maturity, Due after ten years, Amortized Cost	1,461,392
Total held to maturity debt securities, Amortized Cost	3,069,262	$ 2,977,511
Debt Securities Held to Maturity, Due in one year or less, Fair Value	24,909
Debt Securities Held to Maturity, Due after one year through five years, Fair Value	127,950
Debt Securities Held to Maturity, Due after five years through ten years, Fair Value	1,407,279
Debt Securities Held to Maturity, Due after ten years, Fair Value	1,415,317
Total held to maturity debt securities, Fair Value	$ 2,975,455	$ 2,962,010