Earnings per Share/Capital Stock: (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Basic
Net income	$ 12,149		$ 11,021
Less: Distributed earnings to common shareholders	8,138		7,846
Less: Distributed earnings to participating securities	45		43
Undistributed earnings	3,966		3,132
Undistributed earnings allocated to common shareholders	3,945		3,114
Undistributed earnings allocated to participating securities	21		18
Total income available to common shareholders, basic	12,083		10,960
Weighted average Common Shares outstanding, basic (in shares)	38,205		38,747
Basic earnings per Common Share (in dollars per share)	$ 0.32		$ 0.28
Diluted
Total income available to common shareholders, basic	12,083		10,960
Undistributed earnings for dilutive stock options	21		18
Total common shareholders earnings, diluted	$ 12,104		$ 10,978
Weighted average Common Shares outstanding, basic (in shares)	38,205		38,747
Stock-based compensation (in shares)	203	[1]	197	[1]
Weighted average common shares outstanding, diluted (in shares)	38,408		38,944
Diluted earnings per Common Share (in dollars per share)	$ 0.32		$ 0.28

[1] In applying the treasury stock method of reflecting the dilutive effect of outstanding stock-based compensation in the calculation of diluted EPS, 198,764 and 247,064 stock options at March 31, 2015 and 2014, respectively, were deemed to be outstanding in accordance with accounting guidance on earnings per share. All of the 226,319 and 237,174 restricted stock units at March 31, 2015 and 2014, respectively, were included in the calculation of diluted EPS for the three months ended March 31, 2015 and 2014.