Equity Based Compensation Plans, Restricted stock activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restricted Stock [Member]
Shares [Roll forward]
Unvested at beginning of year (in shares)	58,349	54,676	25,820
Granted (in shares)	62,706	49,183	56,177
Vested (in shares)	(29,675)	(38,427)	(22,939)
Forfeited and canceled (in shares)	(557)	(7,083)	(4,382)
Unvested at end of year (in shares)	90,823	58,349	54,676	25,820
Weighted-average grant price [Abstract]
Unvested at beginning of year (in dollars per share)	$ 149.11	$ 147.10	$ 168.22
Granted (in dollars per share)	177.28	150.13	155.21
Vested (in dollars per share)	170.17	147.12	148.20
Forfeited and canceled (in dollars per share)	119.37	141.76	122.06
Unvested at end of year (in dollars per share)	$ 163.49	$ 149.11	$ 147.10	$ 168.22
Long Term Incentive Plans - Z Units and 2014 LTIP Units [Member]
Shares [Roll forward]
Unvested at beginning of year (in shares)	60,027	106,157	151,067
Granted (in shares)	0	0	0
Vested (in shares)	(32,961)	(36,842)	(36,650)
Converted (in shares)	0	0	0
Forfeited and canceled (in shares)	(3,854)	(9,288)	(8,260)
Unvested at end of year (in shares)	23,212	60,027	106,157	151,067
Weighted-average grant price [Abstract]
Unvested at beginning of year (in dollars per share)	$ 75.11	$ 75.41	$ 71.14
Unvested at end of year (in dollars per share)	$ 75.03	$ 75.11	$ 75.41	$ 71.14
Incentive units vested [Roll Forward]
Vested, beginning balance (in shares)	181,027	144,185	181,919
Vested, units granted (in shares)	0	0	0
Vested, units vested (in shares)	32,961	36,842	36,650
Converted (in shares)	(688)	0	(74,384)
Cancelled (in shares)	0	0	0
Vested, ending balance (in shares)	213,300	181,027	144,185	181,919
Total outstanding incentive units [Abstract]
Total outstanding units, beginning balance (in shares)	241,054	250,342	332,986
Total outstanding units, granted (in shares)	0	0	0
Total outstanding units, vested (in shares)	0	0	0
Total outstanding units, converted (in shares)	(688)	0	(74,384)
Total outstanding units, cancelled (in shares)	(3,854)	(9,288)	(8,260)
Total outstanding units, ending balance (in shares)	236,512	241,054	250,342	332,986
Weighted average remaining contractual life outstanding units	7 years 6 months	8 years 6 months	9 years 6 months	10 years 6 months