Partners' Capital, Mezzanine Equity and Distributions (Narrative) (Details) $ / shares in Units, $ in Thousands			1 Months Ended		6 Months Ended
	Aug. 14, 2018 $ / shares shares	Sep. 01, 2017 USD ($) $ / shares shares	May 01, 2018 USD ($) shares	Jan. 31, 2018 USD ($) shares	Jun. 30, 2018 USD ($) shares	Dec. 31, 2017 shares
Temporary Equity [Line Items]
Number of convertible preferred units issued (in shares)					23,402,956	22,411,728
Partners' capital, units outstanding (in shares)					122,579,218	122,579,218
Class A Convertible Preferred Stock Units
Temporary Equity [Line Items]
Distribution paid-in-kind | $			$ 16,900	$ 16,500	$ 33,414
Distributions paid-in-kind, shares (in shares)			500,976	490,252	991,228
Preferred units, cumulative cash distributions | $					$ 34,100
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
Partners’ Capital | Common Class A
Temporary Equity [Line Items]
Partners' capital, units outstanding (in shares)					122,539,221
Partners’ Capital | Common Class B
Temporary Equity [Line Items]
Partners' capital, units outstanding (in shares)					39,997
Subsequent Event | Class A Convertible Preferred Stock Units | Quarterly Rate
Temporary Equity [Line Items]
Stock dividend rate per unit (in dollars per share) | $ / shares	$ 0.7374
Subsequent Event | Class A Convertible Preferred Stock Units | In Arrears At Annual Rate
Temporary Equity [Line Items]
Stock dividend rate per unit (in dollars per share) | $ / shares	$ 2.9496
Subsequent Event | Scenario, Forecast | Class A Convertible Preferred Stock Units
Temporary Equity [Line Items]
Distributions paid-in-kind, shares (in shares)	511,934