Earnings per Share/Capital Stock: (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Anti-dilutive stock options not included in the computation of diluted EPS (in shares)							0
Payment, Tax Withholding, Share-based Payment Arrangement							$ 1,857	$ 1,581
Dividends paid							34,848	31,466
Basic
Net income	$ 26,530	$ 25,612	$ 14,072	$ 28,006	$ 26,784	$ 12,852	$ 66,214	$ 67,642
Dividends Declared Per Common Share (in dollars per share)	$ 0.335			$ 0.305			$ 0.945	$ 0.855
Weighted Average Dividends Common Stock	$ 12,356			$ 11,234			$ 34,848	$ 31,466
Less: Distributed earnings to participating securities	43			50			116	137
Undistributed earnings	14,131			16,722			31,250	36,039
Undistributed earnings allocated to common shareholders	14,083			16,647			31,146	35,882
Undistributed earnings allocated to participating securities	48			75			104	157
Total income available to common shareholders, basic	$ 26,439			$ 27,881			$ 65,994	$ 67,348
Weighted average Common Shares outstanding, basic (in shares)	36,886,000			36,835,000			36,877,000	36,804,000
Basic earnings per Common Share (in dollars per share)	$ 0.72			$ 0.76			$ 1.79	$ 1.83
Diluted
Total income available to common shareholders, basic	$ 26,439			$ 27,881			$ 65,994	$ 67,348
Undistributed earnings for dilutive stock-based awards	48			75			104	157
Total common shareholders earnings, diluted	$ 26,487			$ 27,956			$ 66,098	$ 67,505
Weighted average Common Shares outstanding, basic (in shares)	36,886,000			36,835,000			36,877,000	36,804,000
Stock-based compensation (in shares)	116,000			161,000			113,000	156,000
Weighted Average Number of Diluted Shares (in shares)	37,002,000			36,996,000			36,990,000	36,960,000
Diluted earnings per Common Share (in dollars per share)	$ 0.72			$ 0.76			$ 1.79	$ 1.83