Partners' Capital, Mezzanine Equity and Distributions (Narrative) (Details) $ / shares in Units, $ in Thousands			1 Months Ended			9 Months Ended
	Nov. 14, 2018 $ / shares shares	Sep. 01, 2017 USD ($) $ / shares shares	Jul. 31, 2018 USD ($) shares	May 01, 2018 USD ($) shares	Jan. 31, 2018 USD ($) shares	Sep. 30, 2018 USD ($) shares	Dec. 31, 2017 shares
Temporary Equity [Line Items]
Number of convertible preferred units issued (in shares)						23,914,890	22,411,728
Partners' capital, units outstanding (in shares)						122,579,218	122,579,218
Common Class A | Partners’ Capital
Temporary Equity [Line Items]
Partners' capital, units outstanding (in shares)						122,539,221
Common Class B | Partners’ Capital
Temporary Equity [Line Items]
Partners' capital, units outstanding (in shares)						39,997
Class A Convertible Preferred Stock Units
Temporary Equity [Line Items]
Distributions paid-in-kind | $			$ 17,300	$ 16,900	$ 16,500	$ 50,671
Distributions paid-in-kind, shares (in shares)			511,934	500,976	490,252	1,503,162
Preferred units, cumulative cash distributions | $						$ 51,800
Class A Convertible Preferred Stock Units | Subsequent Event | Quarterly Rate
Temporary Equity [Line Items]
Stock dividend rate per unit (in dollars per share) | $ / shares	$ 0.7374
Class A Convertible Preferred Stock Units | Subsequent Event | In Arrears At Annual Rate
Temporary Equity [Line Items]
Stock dividend rate per unit (in dollars per share) | $ / shares	$ 2.9496
Class A Convertible Preferred Stock Units | Subsequent Event | Scenario, Forecast
Temporary Equity [Line Items]
Distributions paid-in-kind, shares (in shares)	523,132
Private Placement | Class A Convertible Preferred Stock Units
Temporary Equity [Line Items]
Temporary equity, stock issued during period (in shares) | $		$ 750,000
Number of convertible preferred units issued (in shares)		22,249,494
Cash purchase price per unit (in dollars per share) | $ / shares		$ 33.71
Number of initial purchasers		2