Equity Based Compensation Plans, Restricted stock activity (Details) (USD $)	12 Months Ended			1 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013	Dec. 31, 2011
Shares [Roll forward]
Unvested at beginning of year (in shares)	16,176	24,922	35,219
Assumed - Merger (in shares)	119,411	0	0
Vested (in shares)	(126,931)	(7,211)	(8,641)
Forfeited and canceled (in shares)	(4,850)	(3,091)	(3,270)
Unvested at end of year (in shares)	25,820	16,176	24,922	16,176
Weighted-average grant price [Abstract]
Unvested at beginning of year (in dollars per share)	$ 108.06	$ 104.52	$ 98.57
Granted (in dollars per share)	$ 194.03	$ 158.75	$ 149.68
Assumed - Merger (in dollars per share)	$ 173.82	$ 0.00	$ 0.00
Vested (in dollars per share)	$ 171.56	$ 109.86	$ 106.69
Forfeited and canceled (in dollars per share)	$ 135.10	$ 100.84	$ 102.00
Unvested at end of year (in dollars per share)	$ 168.22	$ 108.06	$ 104.52	108.06
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Issued During Period, Shares, New Issues	22,014	1,556	1,614
Series Z Incentive Units [Member]
Shares [Roll forward]
Granted (in shares)				50,500	154,500
Long Term Incentive Plans - Z Units and 2014 LTIP Units [Member]
Shares [Roll forward]
Unvested at beginning of year (in shares)	149,381	140,043	170,019
Granted (in shares)	44,750	50,500	0
Vested (in shares)	(41,729)	(35,919)	(28,163)
Converted (in shares)	0	0	0
Forfeited and canceled (in shares)	(1,335)	(5,243)	(1,813)
Unvested at end of year (in shares)	151,067	149,381	140,043	149,381	170,019
Weighted-average grant price [Abstract]
Unvested at beginning of year (in dollars per share)	$ 63.53	$ 58.44	$ 58.17
Unvested at end of year (in dollars per share)	$ 71.14	$ 63.53	$ 58.44	63.53	58.17
Incentive units vested [Roll Forward]
Vested, beginning balance (in shares)	118,190	190,704	179,082
Vested, units granted (in shares)	24,000	0	0
Vested, units vested (in shares)	41,729	35,919	28,163
Converted (in shares)	(2,000)	(108,433)	(16,541)
Cancelled (in shares)	0	0	0
Vested, ending balance (in shares)	181,919	118,190	190,704	118,190	179,082
Total outstanding incentive units [Abstract]
Total outstanding units, beginning balance (in shares)	267,571	330,747	349,101
Total outstanding units, granted (in shares)	68,750	50,500	0
Total outstanding units, vested (in shares)	0	0	0
Total outstanding units, converted (in shares)	(2,000)	(108,433)	(16,541)
Total outstanding units, cancelled (in shares)	(1,335)	(5,243)	(1,813)
Total outstanding units, ending balance (in shares)	332,986	267,571	330,747	267,571	349,101
Weighted-average remaining contractual life, beginning balance	10 years 6 months	9 years 3 months 24 days	11 years 3 months 24 days		12 years 3 months 24 days
Weighted-average remaining contractual life, ending balance	10 years 6 months	9 years 3 months 24 days	11 years 3 months 24 days		12 years 3 months 24 days