Stock option and award plan: (Tables)

12 Months Ended

Dec. 31, 2021

Stock option and award plan:

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

	Years Ended
	December 31,
Black-Scholes Assumptions	2021		2020		2019
Dividend yield	4.6	%	3.4	%	4.5	%
Expected volatility	33.4	%	31.5	%	28.3	%
Risk-free interest rate	0.6	%	1.1	%	2.5	%
Expected life of the option term (in years)	4.2		4.2		4.3

Schedule of stock option activity

	Number of	Weighted-Average
	Options	Exercise Price
Outstanding at December 31, 2020	159,880	$	54.09
Granted	76,768	$	69.54
Cancelled and expired	(51,645)	$	68.00
Exercised—intrinsic value $0.9 million; cash received $1.8 million	(36,468)	$	50.00
Outstanding at December 31, 2021—$2.4 million intrinsic value and 6.9 years weighted-average remaining contractual term	148,535	$	58.24
Exercisable at December 31, 2021—$2.1 million intrinsic value and 5.5 years weighted-average remaining contractual term	89,904	$	50.70
Expected to vest—$2.3 million intrinsic value and 6.6 years weighted-average remaining contractual term	131,701	$	56.55

Schedule of non-vested restricted stock awards

		Weighted-Average
		Grant Date
Non-vested awards	Shares	Fair Value
Non-vested at December 31, 2020	1,339,596	$	55.70
Granted	471,080	$	64.59
Vested	(509,919)	$	49.12
Forfeited	(47,436)	$	57.16
Non-vested at December 31, 2021	1,253,321	$	61.66

	Years Ended
	December 31,
Additional Award Plan Information – Related to Stock Options & Restricted Stock (thousands)	2021		2020		2019
Equity-based compensation expense	$	26,822	$	23,525	$	18,460
Income tax benefit related to stock options and restricted stock		6,314		4,211		3,026
Capitalized compensation expense related to stock options and restricted stock		3,222		2,275		1,752
Intrinsic value of stock options exercised		881		841		839
Fair value of shares of restricted stock vested		35,749		25,439		20,779