Earnings per Share/Capital Stock: (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Basic
Net income	$ 13,465	$ 10,115
Less: Distributed earnings to common shareholders	6,839	5,273
Less: Distributed earnings to participating securities	107	35
Undistributed earnings	6,519	4,807
Undistributed earnings allocated to common shareholders	6,419	4,775
Undistributed earnings allocated to participating securities	100	32
Total income available to common shareholders, basic	13,258	10,048
Weighted average Common Shares outstanding, basic	19,265,000	18,831,000
Basic earnings per Common Share (in dollars per share)	$ 0.69	$ 0.53
Stock option and restricted stock option outstanding
Options outstanding (in shares)	176,048	566,406
Diluted
Common shareholders earnings, basic	13,258	10,048
Undistributed earnings for dilutive stock options		32
Total common shareholders earnings, diluted	$ 13,258	$ 10,080
Weighted average Common Shares outstanding, basic	19,265,000	18,831,000
Stock-based compensation (in shares)	46,000	142,000
Weighted average common shares outstanding, diluted	19,311,000	18,973,000
Diluted earnings per Common Share (in dollars per share)	$ 0.69	$ 0.53
Restricted Stock Units
Share based compensation arrangement
Restricted stock units outstanding (in shares)	145,208	147,078