Equity Based Compensation Plans, Restricted stock activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock [Member]
Shares [Roll forward]
Unvested at beginning of year (in shares)	90,823	58,349	54,676
Granted (in shares)	51,945	62,706	49,183
Vested (in shares)	(48,212)	(29,675)	(38,427)
Forfeited and canceled (in shares)	(3,498)	(557)	(7,083)
Unvested at end of year (in shares)	91,058	90,823	58,349	54,676
Weighted-average grant price [Abstract]
Unvested at beginning of year (in dollars per share)	$ 163.49	$ 149.11	$ 147.10
Granted (in dollars per share)	194.70	177.28	150.13
Vested (in dollars per share)	150.76	170.17	147.12
Forfeited and canceled (in dollars per share)	158.71	119.37	141.76
Unvested at end of year (in dollars per share)	$ 180.99	$ 163.49	$ 149.11	$ 147.10
Long Term Incentive Plans - Z Units and 2014 LTIP Units [Member]
Shares [Roll forward]
Unvested at beginning of year (in shares)	23,212	60,027	106,157
Granted (in shares)	0	0	0
Vested (in shares)	(12,051)	(32,961)	(36,842)
Converted (in shares)	0	0	0
Forfeited and canceled (in shares)	0	(3,854)	(9,288)
Unvested at end of year (in shares)	11,161	23,212	60,027	106,157
Weighted-average grant price [Abstract]
Unvested at beginning of year (in dollars per share)	$ 75.03	$ 75.11	$ 75.41
Unvested at end of year (in dollars per share)	$ 75.03	$ 75.03	$ 75.11	$ 75.41
Incentive units vested [Roll Forward]
Vested, beginning balance (in shares)	213,300	181,027	144,185
Vested, units granted (in shares)	0	0	0
Vested, units vested (in shares)	12,051	32,961	36,842
Converted (in shares)	(91,270)	(688)	0
Cancelled (in shares)	0	0	0
Vested, ending balance (in shares)	134,081	213,300	181,027	144,185
Total outstanding incentive units [Abstract]
Total outstanding units, beginning balance (in shares)	236,512	241,054	250,342
Total outstanding units, granted (in shares)	0	0	0
Total outstanding units, vested (in shares)	0	0	0
Total outstanding units, converted (in shares)	(91,270)	(688)	0
Total outstanding units, cancelled (in shares)	0	(3,854)	(9,288)
Total outstanding units, ending balance (in shares)	145,242	236,512	241,054	250,342
Weighted average remaining contractual life outstanding units	6 years 6 months	7 years 6 months	8 years 6 months	9 years 6 months