Earnings per Share/Capital Stock: (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic
Net income		$ 16,348	$ 22,792	$ 27,130	$ 35,493
Weighted Average Dividends Common Stock		9,367	8,861	18,729	17,715
Less: Distributed earnings to participating securities		51	48	96	91
Undistributed earnings		6,930	13,883	8,305	17,687
Undistributed earnings allocated to common shareholders		6,894	13,807	8,263	17,597
Undistributed earnings allocated to participating securities		36	76	42	90
Total income available to common shareholders, basic		$ 16,261	$ 22,668	$ 26,992	$ 35,312
Weighted average Common Shares outstanding, basic (in shares)		36,733	36,624	36,723	36,607
Basic earnings per Common Share (in dollars per share)		$ 0.44	$ 0.62	$ 0.74	$ 0.96
Diluted
Total income available to common shareholders, basic		$ 16,261	$ 22,668	$ 26,992	$ 35,312
Undistributed earnings for dilutive stock-based awards		36	76	42	90
Total common shareholders earnings, diluted		$ 16,297	$ 22,744	$ 27,034	$ 35,402
Weighted average Common Shares outstanding, basic (in shares)		36,733	36,624	36,723	36,607
Stock-based compensation (in shares)	[1]	179	201	173	192
Weighted average common shares outstanding, diluted (in shares)		36,912	36,825	36,896	36,799
Diluted earnings per Common Share (in dollars per share)		$ 0.44	$ 0.62	$ 0.73	$ 0.96

[1] In applying the treasury stock method of reflecting the dilutive effect of outstanding stock-based compensation in the calculation of diluted EPS, 47,792 and 91,634 stock options at June 30, 2018 and 2017, respectively, were deemed to be outstanding in accordance with the accounting guidance on earnings per share. All of the 204,909 and 208,059 restricted stock units at June 30, 2018 and 2017, respectively, were included in the calculation of diluted EPS for the three and six months ended June 30, 2018 and 2017.