Supplemental Guarantor / Non-Guarantor Financial Information Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues:
Affiliates	$ 106,027			$ 100,188			$ 311,755	$ 295,629
Third Parties	29,868			25,596			89,388	77,799
Revenue from Contract with Customer	135,895			125,784			401,143	373,428
Operating costs and expenses [Abstract]
Operations (exclusive of depreciation and amortization)	44,924			35,996			123,045	106,731
Depreciation and amortization	24,121			24,367			72,192	74,117
General and administrative	2,714			2,498			7,322	8,293
Total operating costs and expenses	71,759			62,861			202,559	189,141
Operating Income (Loss)	64,136			62,923			198,584	184,287
Equity in earnings of subsidiaries	0			0			0	0
Equity in earnings of equity method investments	1,334			1,114			5,217	4,127
Interest expense	(18,807)			(18,042)			(57,059)	(53,249)
Interest income	2,243			540			3,322	1,581
Gain on sale of assets and other	142			38			(57)	71
Other Nonoperating Income (Expense)	35,166						35,166
Nonoperating Income (Expense)	20,078			(16,350)			(13,411)	(47,470)
Income before income taxes	84,214			46,573			185,173	136,817
State income tax benefit (expense)	(30)			(39)			(36)	(149)
Net income	84,184	$ 47,159	$ 53,794	46,534	$ 41,499	$ 48,635	185,137	136,668
Allocation of net income attributable to noncontrolling interests	(1,839)			(1,531)			(5,920)	(5,354)
Net income attributable to the partners	82,345			45,003			179,217	131,314
Parent
Revenues:
Affiliates	0			0			0	0
Third Parties	0			0			0	0
Revenue from Contract with Customer	0			0			0	0
Operating costs and expenses [Abstract]
Operations (exclusive of depreciation and amortization)	0			0			0	0
Depreciation and amortization	0			0			0	0
General and administrative	569			698			2,390	2,739
Total operating costs and expenses	569			698			2,390	2,739
Operating Income (Loss)	(569)			(698)			(2,390)	(2,739)
Equity in earnings of subsidiaries	101,638			63,731			238,368	187,349
Equity in earnings of equity method investments	0			0			0	0
Interest expense	(18,945)			(18,030)			(56,982)	(53,341)
Interest income	0			0			0	0
Gain on sale of assets and other	221			0			221	45
Other Nonoperating Income (Expense)	0						0
Nonoperating Income (Expense)	82,914			45,701			181,607	134,053
Income before income taxes	82,345			45,003			179,217	131,314
State income tax benefit (expense)	0			0			0	0
Net income	82,345			45,003			179,217	131,314
Allocation of net income attributable to noncontrolling interests	0			0			0	0
Net income attributable to the partners	82,345			45,003			179,217	131,314
Guarantor Restricted Subsidiaries
Revenues:
Affiliates	99,482			94,270			293,096	278,083
Third Parties	23,999			21,277			69,764	60,795
Revenue from Contract with Customer	123,481			115,547			362,860	338,878
Operating costs and expenses [Abstract]
Operations (exclusive of depreciation and amortization)	40,866			32,906			111,644	97,064
Depreciation and amortization	19,757			20,198			59,320	61,630
General and administrative	2,145			1,800			4,932	5,554
Total operating costs and expenses	62,768			54,904			175,896	164,248
Operating Income (Loss)	60,713			60,643			186,964	174,630
Equity in earnings of subsidiaries	3,013			2,251			10,572	9,344
Equity in earnings of equity method investments	1,334			1,114			5,217	4,127
Interest expense	138			(12)			(77)	92
Interest income	2,243			540			3,322	1,581
Gain on sale of assets and other	(104)			14			(364)	(37)
Other Nonoperating Income (Expense)	35,166						35,166
Nonoperating Income (Expense)	41,790			3,907			53,836	15,107
Income before income taxes	102,503			64,550			240,800	189,737
State income tax benefit (expense)	(30)			(39)			(36)	(149)
Net income	102,473			64,511			240,764	189,588
Allocation of net income attributable to noncontrolling interests	(835)			(780)			(2,396)	(2,239)
Net income attributable to the partners	101,638			63,731			238,368	187,349
Non-Guarantor Non-Restricted Subsidiaries
Revenues:
Affiliates	6,545			5,918			18,659	17,546
Third Parties	5,869			4,319			19,624	17,004
Revenue from Contract with Customer	12,414			10,237			38,283	34,550
Operating costs and expenses [Abstract]
Operations (exclusive of depreciation and amortization)	4,058			3,090			11,401	9,667
Depreciation and amortization	4,364			4,169			12,872	12,487
General and administrative	0			0			0	0
Total operating costs and expenses	8,422			7,259			24,273	22,154
Operating Income (Loss)	3,992			2,978			14,010	12,396
Equity in earnings of subsidiaries	0			0			0	0
Equity in earnings of equity method investments	0			0			0	0
Interest expense	0			0			0	0
Interest income	0			0			0	0
Gain on sale of assets and other	25			24			86	63
Other Nonoperating Income (Expense)	0						0
Nonoperating Income (Expense)	25			24			86	63
Income before income taxes	4,017			3,002			14,096	12,459
State income tax benefit (expense)	0			0			0	0
Net income	4,017			3,002			14,096	12,459
Allocation of net income attributable to noncontrolling interests	(1,004)			(751)			(3,524)	(3,115)
Net income attributable to the partners	3,013			2,251			10,572	9,344
Eliminations
Revenues:
Affiliates	0			0			0	0
Third Parties	0			0			0	0
Revenue from Contract with Customer	0			0			0	0
Operating costs and expenses [Abstract]
Operations (exclusive of depreciation and amortization)	0			0			0	0
Depreciation and amortization	0			0			0	0
General and administrative	0			0			0	0
Total operating costs and expenses	0			0			0	0
Operating Income (Loss)	0			0			0	0
Equity in earnings of subsidiaries	(104,651)			(65,982)			(248,940)	(196,693)
Equity in earnings of equity method investments	0			0			0	0
Interest expense	0			0			0	0
Interest income	0			0			0	0
Gain on sale of assets and other	0			0			0	0
Other Nonoperating Income (Expense)	0						0
Nonoperating Income (Expense)	(104,651)			(65,982)			(248,940)	(196,693)
Income before income taxes	(104,651)			(65,982)			(248,940)	(196,693)
State income tax benefit (expense)	0			0			0	0
Net income	(104,651)			(65,982)			(248,940)	(196,693)
Allocation of net income attributable to noncontrolling interests	0			0			0	0
Net income attributable to the partners	$ (104,651)			$ (65,982)			$ (248,940)	$ (196,693)