Stock option and award plan: (Tables)

12 Months Ended

Dec. 31, 2016

Stock option and award plan:

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

	Years Ended December 31,
Black-Scholes Assumptions	2016			2015			2014
Dividend yield		3.7	%		3.8	%		3.6	%
Expected volatility		30.8	%		32.5	%		37.0	%
Risk-free interest rate		1.4	%		1.5	%		1.6	%
Expected life of the option term (in years)		4.6			4.6			4.6

Schedule of stock option activity

	Number of Options			Weighted-Average Exercise Price
Outstanding at December 31, 2015		201,587		$	28.42
Granted		72,752		$	37.54
Cancelled and expired		(37,772	)	$	34.66
Exercised—intrinsic value $0.9 million; cash received $1.2 million		(53,245	)	$	22.92
Outstanding at December 31, 2016—$1.7 million intrinsic value and 7.4 years weighted-average remaining contractual term		183,322		$	32.35
Exercisable at December 31, 2016—$1.2 million intrinsic value and 6.2 years weighted-average remaining contractual term		103,898		$	29.61
Expected to vest—$1.5 million intrinsic value and 7.1 years weighted-average remaining contractual term		162,235		$	31.85

Schedule of non-vested restricted stock awards

Non-vested awards	Shares			Weighted-Average Grant Date Fair Value
Non-vested at December 31, 2015		870,751		$	32.25
Granted		358,130		$	38.87
Vested		(316,296	)	$	29.07
Forfeited		(3,824	)	$	37.27
Non-vested at December 31, 2016		908,761		$	35.94