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Share and Unit-Based Plans - Nonvested equity awards (Details)	9 Months Ended
	Sep. 30, 2018 $ / shares shares
LTIP Units
Units
Balance at beginning of period (in shares) | shares	636,632
Granted (in shares) | shares	569,136
Vested (in shares) | shares	(189,044)
Forfeited (in shares) | shares	(23,666)
Balance at end of period (in shares) | shares	993,058
Value
Balance at beginning of period (in dollars per share) | $ / shares	$ 52.36
Granted (in dollars per share) | $ / shares	51.78
Vested (in dollars per share) | $ / shares	57.49
Forfeited (in dollars per share) | $ / shares	44.28
Balance at end of period (in dollars per share) | $ / shares	$ 51.24
Phantom Stock Units
Units
Balance at beginning of period (in shares) | shares	4,054
Granted (in shares) | shares	8,765
Vested (in shares) | shares	(10,581)
Forfeited (in shares) | shares	(845)
Balance at end of period (in shares) | shares	1,393
Value
Balance at beginning of period (in dollars per share) | $ / shares	$ 79.82
Granted (in dollars per share) | $ / shares	61.46
Vested (in dollars per share) | $ / shares	54.70
Forfeited (in dollars per share) | $ / shares	77.91
Balance at end of period (in dollars per share) | $ / shares	$ 66.79
Stock Units
Units
Balance at beginning of period (in shares) | shares	151,355
Granted (in shares) | shares	87,193
Vested (in shares) | shares	(108,201)
Forfeited (in shares) | shares	0
Balance at end of period (in shares) | shares	130,347
Value
Balance at beginning of period (in dollars per share) | $ / shares	$ 73.32
Granted (in dollars per share) | $ / shares	58.85
Vested (in dollars per share) | $ / shares	74.21
Forfeited (in dollars per share) | $ / shares	0.00
Balance at end of period (in dollars per share) | $ / shares	$ 64.24

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
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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.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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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.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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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.

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