Long-term Debt and Short-term Borrowings (Components of Short-term Borrowings) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Total balance at December 31,	$ 161,190	$ 159,699	$ 177,025
Weighted average interest rate at December 31,	0.18%	0.08%	0.08%
Maximum month end balance during the year	$ 225,475	$ 286,175	$ 250,453
Average amount outstanding during the year	$ 184,093	$ 216,593	$ 205,305
Weighted average interest rate during the year	0.11%	0.09%	0.08%