Unit Option Activity (Details) - Employee Stock Option [Member] $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Unit options, outstanding (in shares) | shares	374
Unit options, exercised (in shares) | shares	(6)
Unit options, outstanding (in shares) | shares	368
Unit options exercisable (in shares) | shares	368
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Unit options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 34.55
Unit options exercised, weighted average exercise price (in dollars per share) | $ / shares	36.95
Unit options outstanding, weighted average exercise price (in dollars per share) | $ / shares	34.51
Unit options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 34.51
Weighted Average Remaining Contractual Life	3 years 10 months
Aggregate intrinsic value | $	$ 4,707