Supplemental Guarantor / Non-Guarantor Financial Information Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015		Jun. 30, 2016	Jun. 30, 2015
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities				$ 114,192	$ 110,254
Revenues [Abstract]
Affiliates	$ 79,179	$ 68,297		162,025	140,552
Third parties	15,718	15,182		34,882	32,683
Total revenues	94,897	83,479		196,907	173,235
Operating costs and expenses [Abstract]
Operations (exclusive of depreciation and amortization)	27,255	25,400	[1]	54,177	53,465	[1]
Depreciation and amortization	15,709	15,179	[1]	32,260	29,977	[1]
General and administrative	2,863	2,696		5,954	5,986
Total operating costs and expenses	45,827	43,275	[1]	92,391	89,428	[1]
Operating income	49,070	40,204	[1]	104,516	83,807	[1]
Equity in Earnings (Loss) of Subsidiaries	0	0		0	0
Equity in Earnings of Equity Method Investments	3,623	631		6,388	1,365
Interest expense	(11,276)	(9,056)		(21,811)	(17,824)
Interest income	112	3		224	3
Gain (Loss) on Sale of Assets	(5)	50		(5)	209
Other (income) expense	(5)	(21)		3	(21)
Nonoperating Income (Expense)	(7,541)	(8,351)		(15,207)	(16,226)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	41,529	31,853	[1]	89,309	67,581	[1]
State income tax (expense) benefit	(54)	64		(149)	(37)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	41,475	31,917	[1]	89,160	67,544	[1]
Allocation of Net Income Attributable to Noncontrolling Interest	(2,355)	(1,743)		(7,282)	(5,770)
Net Income (Loss) Attributable to Parent	39,120	30,174		81,878	61,774
Other comprehensive income (loss)	(142)	222		(595)	(527)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	38,978	30,396	[1]	81,283	61,247	[1]
Equity Method Investments [Member]
Operating costs and expenses [Abstract]
Equity in Earnings of Equity Method Investments	3,623	631		6,388	1,365
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities				(10,608)	(9,639)
Revenues [Abstract]
Affiliates	0	0		0	0
Third parties	0	0		0	0
Total revenues	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
General and administrative	971	637		2,136	1,700
Total operating costs and expenses	971	637		2,136	1,700
Operating income	(971)	(637)		(2,136)	(1,700)
Equity in Earnings (Loss) of Subsidiaries	45,164	36,111		94,154	74,044
Interest expense	(5,073)	(5,073)		(10,140)	(10,140)
Interest income	0	0		0	0
Gain (Loss) on Sale of Assets	0	0		0	0
Other (income) expense	0	0		0	0
Nonoperating Income (Expense)	40,091	31,038		84,014	63,904
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	39,120	30,401		81,878	62,204
State income tax (expense) benefit	0	0		0	0
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	39,120	30,401		81,878	62,204
Allocation of Net Income Attributable to Noncontrolling Interest	0	0		0	0
Net Income (Loss) Attributable to Parent	39,120	30,401		81,878	62,204
Other comprehensive income (loss)	(142)	222		(595)	(527)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	38,978	30,623		81,283	61,677
Parent Company [Member] | Equity Method Investments [Member]
Operating costs and expenses [Abstract]
Equity in Earnings of Equity Method Investments	0	0		0	0
Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities				113,145	111,307
Revenues [Abstract]
Affiliates	74,787	64,727		147,039	127,783
Third parties	11,691	11,895		22,423	23,282
Total revenues	86,478	76,622		169,462	151,065
Operating costs and expenses [Abstract]
Operations (exclusive of depreciation and amortization)	24,444	22,222		48,335	47,852
Depreciation and amortization	11,922	11,432		24,715	22,498
General and administrative	1,892	2,059		3,818	4,286
Total operating costs and expenses	38,258	35,713		76,868	74,636
Operating income	48,220	40,909		92,594	76,429
Equity in Earnings (Loss) of Subsidiaries	1,370	(88)		10,553	6,772
Interest expense	(6,203)	(3,983)		(11,671)	(7,684)
Interest income	107	3		212	3
Gain (Loss) on Sale of Assets	(5)	50		(5)	209
Other (income) expense	(5)	(70)		4	(70)
Nonoperating Income (Expense)	(1,103)	(3,317)		5,473	735
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	47,117	37,592		98,067	77,164
State income tax (expense) benefit	(54)	64		(149)	(37)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	47,063	37,656		97,918	77,127
Allocation of Net Income Attributable to Noncontrolling Interest	0	0		0	0
Net Income (Loss) Attributable to Parent	47,063	37,656		97,918	77,127
Other comprehensive income (loss)	(142)	222		(595)	(527)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	46,921	37,878		97,323	76,600
Guarantor Subsidiaries [Member] | Equity Method Investments [Member]
Operating costs and expenses [Abstract]
Equity in Earnings of Equity Method Investments	3,623	631		6,388	1,365
Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities				19,155	15,358
Revenues [Abstract]
Affiliates	4,392	3,545		14,986	12,769
Third parties	4,027	3,287		12,459	9,401
Total revenues	8,419	6,832		27,445	22,170
Operating costs and expenses [Abstract]
Operations (exclusive of depreciation and amortization)	2,811	3,153		5,842	5,613
Depreciation and amortization	3,787	3,747		7,545	7,479
General and administrative	0	0		0	0
Total operating costs and expenses	6,598	6,900		13,387	13,092
Operating income	1,821	(68)		14,058	9,078
Equity in Earnings (Loss) of Subsidiaries	0	0		0	0
Interest expense	0	0		0	0
Interest income	5	0		12	0
Gain (Loss) on Sale of Assets	0	0		0	0
Other (income) expense	0	49		(1)	49
Nonoperating Income (Expense)	5	(49)		13	(49)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	1,826	(117)		14,071	9,029
State income tax (expense) benefit	0	0		0	0
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	1,826	(117)		14,071	9,029
Allocation of Net Income Attributable to Noncontrolling Interest	0	0		0	0
Net Income (Loss) Attributable to Parent	1,826	(117)		14,071	9,029
Other comprehensive income (loss)	0	0		0	0
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	1,826	(117)		14,071	9,029
Non-Guarantor Subsidiaries [Member] | Equity Method Investments [Member]
Operating costs and expenses [Abstract]
Equity in Earnings of Equity Method Investments	0	0		0	0
Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities				(7,500)	(6,772)
Revenues [Abstract]
Affiliates	0	25		0	0
Third parties	0	0		0	0
Total revenues	0	25		0	0
Operating costs and expenses [Abstract]
Operations (exclusive of depreciation and amortization)	0	25		0	0
Depreciation and amortization	0	0		0	0
General and administrative	0	0		0	0
Total operating costs and expenses	0	25		0	0
Operating income	0	0		0	0
Equity in Earnings (Loss) of Subsidiaries	(46,534)	(36,023)		(104,707)	(80,816)
Interest expense	0	0		0	0
Interest income	0	0		0	0
Gain (Loss) on Sale of Assets	0	0		0	0
Other (income) expense	0	0		0	0
Nonoperating Income (Expense)	(46,534)	(36,023)		(104,707)	(80,816)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(46,534)	(36,023)		(104,707)	(80,816)
State income tax (expense) benefit	0	0		0	0
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	(46,534)	(36,023)		(104,707)	(80,816)
Allocation of Net Income Attributable to Noncontrolling Interest	(2,355)	(1,743)		(7,282)	(5,770)
Net Income (Loss) Attributable to Parent	(48,889)	(37,766)		(111,989)	(86,586)
Other comprehensive income (loss)	142	(222)		595	527
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	(48,747)	(37,988)		(111,394)	(86,059)
Eliminations [Member] | Equity Method Investments [Member]
Operating costs and expenses [Abstract]
Equity in Earnings of Equity Method Investments	$ 0	$ 0		$ 0	$ 0

[1] Retrospectively adjusted as described in Note 1.