Earnings Per Unit - Computation of Basic and Diluted Earnings per Unit (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share Basic [Line Items]
Net income (loss)	$ 60,775	$ 22,034	$ 131,422	$ 137,793
Net (income) loss attributable to noncontrolling interests	(22)	20	(1)	27
Distributions on Series A redeemable preferred units	0	(666)	(25)	(2,452)
Distributions on Series B cumulative convertible preferred units	(5,250)	0	(15,750)	0
NET INCOME (LOSS) ATTRIBUTABLE TO THE GENERAL PARTNER AND COMMON AND SUBORDINATED UNITS	55,503	21,388	115,646	135,368
ALLOCATION OF NET INCOME (LOSS):
General partner interest	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO THE GENERAL PARTNER AND COMMON AND SUBORDINATED UNITS	55,503	21,388	115,646	135,368
Common Units
ALLOCATION OF NET INCOME (LOSS):
Allocation of net income (loss)	$ 29,188	$ 16,371	$ 71,037	$ 83,989
NET INCOME (LOSS) ATTRIBUTABLE TO LIMITED PARTNERS PER COMMON AND SUBORDINATED UNIT:
Per unit (basic) (in dollars per share)	$ 0.27	$ 0.16	$ 0.67	$ 0.86
Weighted average units outstanding (basic) (in shares)	106,706	101,623	105,254	97,777
Per unit (diluted) (in dollars per share)	$ 0.27	$ 0.16	$ 0.67	$ 0.86
Weighted average units outstanding (diluted) (in shares)	106,706	101,623	105,254	97,777
Subordinated Units
ALLOCATION OF NET INCOME (LOSS):
Allocation of net income (loss)	$ 26,315	$ 5,017	$ 44,609	$ 51,379
NET INCOME (LOSS) ATTRIBUTABLE TO LIMITED PARTNERS PER COMMON AND SUBORDINATED UNIT:
Per unit (basic) (in dollars per share)	$ 0.27	$ 0.05	$ 0.46	$ 0.54
Weighted average units outstanding (basic) (in shares)	96,329	95,388	96,021	95,269
Per unit (diluted) (in dollars per share)	$ 0.27	$ 0.05	$ 0.46	$ 0.54
Weighted average units outstanding (diluted) (in shares)	96,329	95,388	96,021	95,269