Earnings per Share/Capital Stock: (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Anti-dilutive stock options not included in the computation of diluted EPS (in shares)					0	0
Payment, Tax Withholding, Share-based Payment Arrangement					$ 1,809	$ 1,573
Basic
Net income	$ 25,612	$ 14,072	$ 26,784	$ 12,852	$ 39,684	$ 39,636
Dividends Declared Per Common Share (in dollars per share)	$ 0.305		$ 0.275		$ 0.610	$ 0.550
Weighted Average Dividends Common Stock	$ 11,250		$ 10,119		$ 22,492	$ 20,232
Less: Distributed earnings to participating securities	40		46		77	88
Undistributed earnings	14,322		16,619		17,115	19,316
Undistributed earnings allocated to common shareholders	14,271		16,543		17,057	19,232
Undistributed earnings allocated to participating securities	51		76		58	84
Total income available to common shareholders, basic	$ 25,521		$ 26,662		$ 39,549	$ 39,464
Weighted average Common Shares outstanding, basic (in shares)	36,884,000		36,804,000		36,872,000	36,788,000
Basic earnings per Common Share (in dollars per share)	$ 0.69		$ 0.72		$ 1.07	$ 1.07
Diluted
Total income available to common shareholders, basic	$ 25,521		$ 26,662		$ 39,549	$ 39,464
Undistributed earnings for dilutive stock-based awards	51		75		58	84
Total common shareholders earnings, diluted	$ 25,572		$ 26,737		$ 39,607	$ 39,548
Weighted average Common Shares outstanding, basic (in shares)	36,884,000		36,804,000		36,872,000	36,788,000
Stock-based compensation (in shares)	116,000		159,000		113,000	154,000
Weighted average common shares outstanding, diluted (in shares)	37,000,000		36,963,000		36,985,000	36,942,000
Diluted earnings per Common Share (in dollars per share)	$ 0.69		$ 0.72		$ 1.07	$ 1.07