Earnings per Share/Capital Stock: (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Anti-dilutive stock options not included in the computation of diluted EPS (in shares)		0
Payments Related to Tax Withholding for Share-based Compensation	$ 1,809	$ 1,569
Basic
Net income	$ 14,072	$ 12,852
Dividends Declared Per Common Share (in dollars per share)	$ 0.305	$ 0.275
Weighted Average Dividends Common Stock	$ 11,242	$ 10,113
Less: Distributed earnings to participating securities	37	47
Undistributed earnings	2,793	2,692
Undistributed earnings allocated to common shareholders	2,784	2,679
Undistributed earnings allocated to participating securities	9	13
Total income available to common shareholders, basic	$ 14,026	$ 12,792
Weighted average Common Shares outstanding, basic (in shares)	36,860,000	36,773,000
Basic earnings per Common Share (in dollars per share)	$ 0.38	$ 0.35
Diluted
Total income available to common shareholders, basic	$ 14,026	$ 12,792
Undistributed earnings for dilutive stock-based awards	9	13
Total common shareholders earnings, diluted	$ 14,035	$ 12,805
Weighted average Common Shares outstanding, basic (in shares)	36,860,000	36,773,000
Stock-based compensation (in shares)	109,000	178,000
Weighted average common shares outstanding, diluted (in shares)	36,969,000	36,951,000
Diluted earnings per Common Share (in dollars per share)	$ 0.38	$ 0.35