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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

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- Definition Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(3) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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