Equity Based Compensation Plans Summary of stock options activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares [Roll Forward]
Beginning balance, outstanding	557,648	525,094	664,785
Granted	164,677	207,429	78,600
Exercised	(176,489)	(138,054)	(203,556)
Forfeited and canceled	(9,628)	(36,821)	(14,735)
Ending balance, outstanding	536,208	557,648	525,094
Options exercisable at year end	223,796	290,340	342,048
Weighted-average exercise price [Abstract]
Outstanding at beginning of year (in dollars per share)	$ 181.50	$ 154.98	$ 138.78
Granted (in dollars per share)	240.56	219.60	227.75
Exercised (in dollars per share)	146.86	138.79	131.53
Forfeited and canceled (in dollars per share)	160.40	178.18	136.11
Outstanding at end of year (in dollars per share)	211.41	181.50	154.98
Options exercisable at year end (in dollars per share)	$ 191.09	$ 160.90	$ 152.42