Equity Based Compensation Plans Summary of stock options activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Shares [Roll Forward]
Beginning balance, outstanding	525,094	664,785	695,488
Granted	207,429	78,600	42,518
Granted - BRE options converted	0	0	133,766
Exercised	(138,054)	(203,556)	(185,387)
Forfeited and canceled	(36,821)	(14,735)	(21,600)
Ending balance, outstanding	557,648	525,094	664,785
Options exercisable at year end	290,340	342,048	395,986
Weighted-average exercise price [Abstract]
Outstanding at beginning of year (in dollars per share)	$ 154.98	$ 138.78	$ 133.37
Granted (in dollars per share)	219.60	227.75	176.65
Granted - BRE options converted (in dollars per share)	0.00	0.00	121.03
Exercised (in dollars per share)	138.79	131.53	113.72
Forfeited and canceled (in dollars per share)	178.18	136.11	144.29
Outstanding at end of year (in dollars per share)	181.50	154.98	138.78
Options exercisable at year end (in dollars per share)	$ 160.90	$ 152.42	$ 133.99