Partners' Capital, Mezzanine and Equity Distributions (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands					12 Months Ended
	Mar. 24, 2017	Jul. 27, 2016	Jul. 22, 2015	Apr. 10, 2015	Dec. 31, 2017	Dec. 31, 2016
Partners Capital And Distributions [Line Items]
Common units outstanding (in shares)					122,579,218	117,979,218
Cash or stock available for distributions, percent usually distributed					100.00%
Days to distribute					45 days
Common units issued (in shares)					122,579,218	117,979,218
Price per share of common units issued (in dollars per share)	$ 30.65	$ 37.90	$ 43.77	$ 44.42
Net proceeds received from sale of units, net of underwriter discounts	$ 140,513	$ 298,452	$ 437,164	$ 198,168
Class A [Member]
Partners Capital And Distributions [Line Items]
Common units issued (in shares)	4,600,000	8,000,000	10,350,000	4,600,000
Price per share of common units issued (in dollars per share)	$ 30.65	$ 37.90	$ 43.77	$ 44.42
Additional common units to purchase (in shares)	600,000		1,350,000	600,000
Net proceeds received from sale of units, net of underwriter discounts	$ 140,500	$ 298,000	$ 437,200	$ 198,200
Class A [Member] | Partners' Capital
Partners Capital And Distributions [Line Items]
Common units outstanding (in shares)					122,539,221
Class B [Member] | Partners' Capital
Partners Capital And Distributions [Line Items]
Common units outstanding (in shares)					39,997