Income Taxes - Narrative (Details) - USD ($)	6 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Tax Credit Carryforward [Line Items]
Federal statutory rate		35.00%
Effective tax rate		24.50%	29.60%	33.60%
Percentage of pre-tax income related to the noncontrolling limited partners' share of earnings		100.00%
Tax benefit in connection with recent IRS guidance		$ (104,675,000)	$ (214,092,000)	$ (175,186,000)
Amortization period		32 years
Effective tax rate, amount		$ 94,100,000	33,900,000	33,300,000
Tax positions for which the ultimate deductibility is highly certain	$ 6,400,000	6,400,000	6,900,000	7,600,000
Interest expense		1,600,000	1,900,000	(400,000)
Interests and penalties	3,600,000	3,600,000	2,000,000	200,000
Deferred tax liability related to net unrealized gain from hedging transactions	40,200,000	40,200,000	141,300,000
Deferred tax asset related to other post-retirement benefits	4,900,000	4,900,000	5,400,000
Deferred tax asset related to the Company’s pension plans	6,100,000	6,100,000	5,100,000
Deferred tax asset	90,300,000	90,300,000	116,000,000
Net of valuation allowances	156,100,000	156,100,000	65,000,000
Reduction of deferred tax assets for uncertain tax positions		31,700,000	300,000
Tax benefits as and addition to common stockholders' equity		13,100,000	26,600,000
Tax benefits for excess tax benefits previously not recorded		8,100,000	6,600,000
Deferred Tax Assets
Tax Credit Carryforward [Line Items]
Uncertain tax positions	114,200,000	114,200,000	$ 10,100,000	$ 9,800,000
Equitable Gas Company, LLC
Tax Credit Carryforward [Line Items]
Tax benefit in connection with recent IRS guidance		35,400,000
EQM
Tax Credit Carryforward [Line Items]
Aggregate offering price (up to)	$ 750,000,000	$ 750,000,000