Income Tax (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current income tax (benefit) expense:
Federal	$ (2,964,000)	$ 15,039,000	$ 5,850,000
State	598,000	2,539,000	1,805,000
Total current	(2,366,000)	17,578,000	7,655,000
Deferred income tax expense (benefit):
Federal	38,531,000	(4,732,000)	(3,870,000)
State	2,163,000	803,000	(596,000)
Total deferred	40,694,000	(3,929,000)	(4,466,000)
Income Tax
Total provision for income tax	38,328,000	13,649,000	3,189,000
Income Tax
Income before provision for income tax	257,116,000	119,894,000	34,291,000
Federal statutory income tax rate (as a percent)	35.00%	35.00%	35.00%
Federal income tax at statutory rate	25,967,000	1,335,000	(2,842,000)
Permanent items	28,000	36,000	20,000
State income taxes net of federal benefit	2,377,000	2,844,000	1,027,000
Current year change in valuation allowance	(5,000)	(64,000)	(1,022,000)
Prior period adjustments and tax rate changes	(2,517,000)	163,000	70,000
Provision on income from Class A units	12,412,000	9,323,000	5,753,000
Other	66,000	12,000	183,000
Provision for income tax	38,328,000	13,649,000	3,189,000
Current deferred tax assets
Accruals and reserves	98,000	78,000
Derivative instruments	5,183,000	14,807,000
Current deferred tax assets	5,281,000	14,885,000
Long-term deferred tax assets
Accruals and reserves	113,000	48,000
Derivative instruments	9,915,000	20,301,000
Phantom unit compensation	2,624,000	2,103,000
Capital loss carryforward	904,000	971,000
State net operating loss carryforward	1,000	101,000
Long-term deferred tax assets	13,557,000	23,524,000
Valuation allowance	(904,000)	(977,000)
Net long-term deferred tax assets	12,653,000	22,547,000
Long-term deferred tax liabilities
Property, plant and equipment and intangibles	3,861,000	2,123,000
Investment in affiliated groups	200,110,000	114,088,000
Long-term deferred tax liabilities	203,971,000	116,211,000
Long-term subtotal	(191,318,000)	(93,664,000)
Net deferred tax liability	(186,037,000)	(78,779,000)
Corporation
Income Tax
Total provision for income tax	36,639,000	10,907,000	1,890,000
Income Tax
Income before provision for income tax	74,192,000	3,813,000	(8,120,000)
Federal statutory income tax rate (as a percent)	35.00%	35.00%	35.00%
Federal income tax at statutory rate	25,967,000	1,335,000	(2,842,000)
Permanent items	28,000	36,000	20,000
State income taxes net of federal benefit	688,000	102,000	(272,000)
Current year change in valuation allowance	(5,000)	(64,000)	(1,022,000)
Prior period adjustments and tax rate changes	(2,517,000)	163,000	70,000
Provision on income from Class A units	12,412,000	9,323,000	5,753,000
Other	66,000	12,000	183,000
Provision for income tax	36,639,000	10,907,000	1,890,000
Long-term deferred tax assets
Capital loss carryforward	900,000
Valuation allowance as a percentage of long-term deferred tax asset attributable to capital loss carryforwards	100.00%
Partnership
Income Tax
Total provision for income tax	1,689,000	2,742,000	1,299,000
Income Tax
Income before provision for income tax	180,640,000	124,087,000	47,761,000
Federal statutory income tax rate (as a percent)	0.00%	0.00%	0.00%
State income taxes net of federal benefit	1,689,000	2,742,000	1,299,000
Provision for income tax	1,689,000	2,742,000	1,299,000
Eliminations
Income Tax
Income before provision for income tax	$ 2,284,000	$ (8,006,000)	$ (5,350,000)
Federal statutory income tax rate (as a percent)	0.00%	0.00%	0.00%