Benefit Plans - LTIP Unit Award Valuation Assumptions (Details) - LTIP Unit Awards	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	2.71%	2.31%
Expected volatility - weighted average	21.68%	20.92%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility - range used	21.50%	20.57%
Risk-free interest rate	0.66%	0.48%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility - range used	21.80%	21.19%
Risk-free interest rate	1.58%	1.43%

Expected dividends to be paid to holders of the underlying shares or financial instruments (expressed as a percentage of the share or instrument's price).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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