Stock option and award plan: (Tables)

12 Months Ended

Dec. 31, 2015

Stock option and award plan:

Schedule of assumptions used for determining the fair value of options granted

	Years Ended December 31,
Black-Scholes Assumptions	2015			2014			2013
Dividend yield		3.8	%		3.6	%		2.0	%
Expected volatility		32.5	%		37.0	%		58.5	%
Risk-free interest rate		1.5	%		1.6	%		0.8	%
Expected life of the option term (in years)		4.6			4.6			4.8

Schedule of stock option activity

	Number of Options			Weighted-Average Exercise Price
Outstanding at December 31, 2014		188,898		$	25.41
Granted		84,008		$	33.50
Cancelled and expired		(43,644	)	$	33.76
Exercised—intrinsic value $0.5 million; cash received $0.4 million		(27,676	)	$	15.29
Outstanding at December 31, 2015—$1.4 million intrinsic value and 6.8 years weighted-average remaining contractual term		201,586		$	28.37
Exercisable at December 31, 2015—$1.2 million intrinsic value and 5.3 years weighted-average remaining contractual term		117,358		$	24.83
Expected to vest—$1.3 million intrinsic value and 6.5 years weighted-average remaining contractual term		179,374		$	27.75

Schedule of non-vested restricted stock awards

Non-vested awards	Shares			Weighted-Average Grant Date Fair Value
Non-vested at December 31, 2014		1,282,646		$	28.26
Granted		62,900		$	33.19
Vested		(427,090	)	$	20.53
Forfeited		(47,705	)	$	31.14
Non-vested at December 31, 2015		870,751		$	32.25