Earnings per Share/Capital Stock: (Details 2) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Additional disclosure
Options outstanding (in shares)	47,792	91,634	47,792	91,634
Anti-dilutive stock options not included in the computation of diluted EPS (in shares)	0		0	0
Common Shares issued under DRP and the 2000 and 2008 Employee Plans			52,712	73,398
Value of Common Shares issued under DRP and the 2000 and 2008 Employee Plans			$ 340	$ 524
Weighted Average Dividends Common Stock	$ 9,367	$ 8,861	18,729	17,715
Dividends paid			$ 18,729	$ 17,715
Quarterly dividends paid, per share of common stock (in dollars per share)	$ 0.255	$ 0.242	$ 0.51	$ 0.484
Restricted Stock Units
Additional disclosure
Restricted stock units outstanding (in shares)	204,909	208,059	204,909	208,059