Borrowings (Short-Term Borrowings) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Short-term repurchase agreement comprised of non-insured customer funds	$ 1,300,000
Federal Funds Purchased And Repurchase Agreements [Member]
Short-term Debt [Line Items]
Year End Weighted Rate	0.80%	0.40%	0.20%
Average Weighted Rate	0.70%	0.30%	0.10%
Average Balance Outstanding	$ 1,462,387	$ 1,266,093	$ 1,654,860
Maximum Outstanding at any Month End	1,984,071	1,723,905	2,193,197
Balance at December 31	$ 1,507,138	$ 1,723,905	$ 1,963,552