Stock option and award plan: (Tables)

12 Months Ended

Dec. 31, 2017

Stock option and award plan:

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

	Years Ended December 31,
Black-Scholes Assumptions	2017			2016			2015
Dividend yield		4.1	%		3.7	%		3.8	%
Expected volatility		27.1	%		30.8	%		32.5	%
Risk-free interest rate		2.0	%		1.4	%		1.5	%
Expected life of the option term (in years)		4.5			4.6			4.6

Schedule of stock option activity

	Number of Options			Weighted-Average Exercise Price
Outstanding at December 31, 2016		183,322		$	32.35
Granted		81,236		$	43.93
Cancelled and expired		(35,340	)	$	38.72
Exercised—intrinsic value $0.5 million; cash received $1.2 million		(39,289	)	$	31.12
Outstanding at December 31, 2017—$1.8 million intrinsic value and 7.5 years weighted-average remaining contractual term		189,929		$	36.37
Exercisable at December 31, 2017—$1.4 million intrinsic value and 6.4 years weighted-average remaining contractual term		109,084		$	32.85
Expected to vest—$1.7 million intrinsic value and 7.3 years weighted-average remaining contractual term		168,762		$	35.63

Schedule of non-vested restricted stock awards

Non-vested awards	Shares			Weighted-Average Grant Date Fair Value
Non-vested at December 31, 2016		908,761		$	35.94
Granted		499,654		$	40.52
Vested		(286,083	)	$	35.35
Forfeited		(10,181	)	$	39.88
Non-vested at December 31, 2017		1,112,151		$	38.11