Equity Based Compensation Plans, Restricted stock activity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Nonvested stock [Roll forward]
Unvested, beginning balance (in shares)	35,219	44,877	37,727
Granted (in shares)	1,614	1,540	14,415
Vested (in shares)	(8,641)	(9,532)	(6,126)
Forfeited and canceled (in shares)	(3,270)	(1,666)	(1,139)
Unvested, ending balance (in shares)	24,922	35,219	44,877
Additional disclosures [Abstract]
Weighted-average grant price, unvested, beginning balance (in dollars per share)	$ 98.57	$ 102.46	$ 99.43
Weighted-average grant price, granted (in dollars per share)	$ 149.68	$ 134.44	$ 109.62
Weighted-average grant price, vested (in dollars per share)	$ 106.69	$ 104.91	$ 102.27
Weighted-average grant price, forfeited and canceled (in dollars per share)	$ 102.00	$ 94.35	$ 93.92
Weighted-average grant price, unvested, ending balance (in dollars per share)	$ 104.52	$ 98.57	$ 102.46
Series Z Incentive Units [Member]
Nonvested stock [Roll forward]
Unvested, beginning balance (in shares)	170,019	171,902	105,881
Granted (in shares)	0	46,500	108,000
Vested (in shares)	(28,163)	(44,520)	(37,629)
Forfeited and canceled (in shares)	(1,813)	(3,863)	(4,350)
Converted (in shares)	0	0
Unvested, ending balance (in shares)	140,043	170,019	171,902
Additional disclosures [Abstract]
Weighted-average grant price, unvested, beginning balance (in dollars per share)	$ 58.17	$ 54.15	$ 39.36
Weighted-average grant price, unvested, ending balance (in dollars per share)	$ 58.44	$ 58.17	$ 54.15
Incentive units vested [Roll Forward]
Vested, beginning balance (in shares)	179,082	326,280	288,651
Vested, units granted (in shares)	0	0	0
Vested, units vested (in shares)	28,163	44,520	37,629
Converted (in shares)	(16,541)	(191,718)
Cancelled (in shares)	0	0
Vested, ending balance (in shares)	190,704	179,082	326,280
Aggregate intrinsic value, unvested, beginning balance	$ 23,719	$ 19,463	$ 8,751
Aggregate intrinsic value, unvested, ending balance	$ 20,800	$ 23,719	$ 19,463
Total outstanding incentive units [Abstract]
Total outstanding units, beginning balance (in shares)	349,101	498,182	394,532
Total outstanding units, granted (in shares)	0	46,500	108,000
Total outstanding units, vested (in shares)	0	0	0
Total outstanding units, converted (in shares)	(16,541)	(191,718)
Total outstanding units, cancelled (in shares)	(1,813)	(3,863)	(4,350)
Total outstanding units, ending balance (in shares)	330,747	349,101	498,182
Weighted-average remaining contractual life, beginning balance	12 years 3 months 18 days	11 years 2 months 12 days	8 years 2 months 12 days
Weighted-average remaining contractual life, ending balance	11 years 3 months 18 days	12 years 3 months 18 days	11 years 2 months 12 days