Earnings per Share/Capital Stock: (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share [Abstract]
Payments Related to Tax Withholding for Share-based Compensation	$ 1,569	$ 1,210
Basic
Net income	$ 12,852	$ 10,782
Dividends Declared Per Common Share (in dollars per share)	$ 0.275	$ 0.255
Weighted Average Dividends Common Stock	$ 10,113	$ 9,362
Less: Distributed earnings to participating securities	47	46
Undistributed earnings	2,692	1,374
Undistributed earnings allocated to common shareholders	2,679	1,367
Undistributed earnings allocated to participating securities	13	7
Total income available to common shareholders, basic	$ 12,792	$ 10,729
Weighted average Common Shares outstanding, basic (in shares)	36,773	36,712
Basic earnings per Common Share (in dollars per share)	$ 0.35	$ 0.29
Diluted
Total income available to common shareholders, basic	$ 12,792	$ 10,729
Undistributed earnings for dilutive stock-based awards	13	8
Total common shareholders earnings, diluted	$ 12,805	$ 10,737
Weighted average Common Shares outstanding, basic (in shares)	36,773	36,712
Stock-based compensation (in shares)	178	162
Weighted average common shares outstanding, diluted (in shares)	36,951	36,874
Diluted earnings per Common Share (in dollars per share)	$ 0.35	$ 0.29