Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	12 Months Ended			0 Months Ended
	Dec. 31, 2014 Segment	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2014	Dec. 29, 2011 GasGatheringSystem	Jul. 16, 2014	Dec. 31, 2011
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 41,871,000	$ 17,229,000	$ 64,994,000				$ 22,000
Allowance for doubtful accounts	0	0
Changes in estimated result of depreciation expense	58,300,000
Changes in estimated result of depreciation expense, per unit	$ 0.29
Changes in estimated result of unconsolidated affiliates	9,400,000
Changes in estimated result of unconsolidated affiliates, per unit	$ 0.05
Changes in estimated result of net income	67,700,000
Changes in estimated result of net income, per unit	$ 0.34
Asset impairment charges	11,700,000
Loss on impairments and disposals of assets	23,783,000
Number of operating segments	8
Liabilities for loss contingencies	0	0
Stock-based compensation expense	113,800,000	35,000,000	9,000,000
Cash payments to MICP participants	88,800,000
Stock Granted, Value, Share-based Compensation, Gross	6,000,000
Weighted Average Period of Unrecognized Share Based Compensation	2 years 3 months 18 days
Amortization expense	23,700,000	24,000,000	11,300,000
Carrying value of the Partnership’s intangible assets	348,700,000	372,400,000
Accumulated amortization, net	69,700,000	46,000,000
Intangible asset amortization in year one	23,600,000
Intangible asset amortization in year two	23,600,000
Intangible asset amortization in year three	23,600,000
Intangible asset amortization in year four	23,600,000
Intangible asset amortization in year five	23,600,000
Long Term Incentive Plan
Summary Of Significant Accounting Policies [Line Items]
Stock-based compensation expense				38,500,000
Common units to be awarded	3.5
Unrecognized compensation expense attributable to LTIP	65,200,000
Management Incentive Compensation Plan
Summary Of Significant Accounting Policies [Line Items]
Stock-based compensation expense	41,100,000
Fair Value, Inputs, Level 3
Summary Of Significant Accounting Policies [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure	1,100,000
Loss on impairments and disposals of assets	$ 23,700,000
Marcellus Shale
Summary Of Significant Accounting Policies [Line Items]
Number of gas gathering system	11				11
Marcellus Shale | Customer Relationships
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life of customer relationship					15 years
Springridge Gathering System | Customer Relationships
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life of customer relationship	15 years
Appalachia Midstream | Customer Relationships
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life of customer relationship	20 years
Minimum
Summary Of Significant Accounting Policies [Line Items]
Award vesting period	4 years					1 year
Minimum | Long Term Incentive Plan
Summary Of Significant Accounting Policies [Line Items]
Unrecognized compensation expense attributable to LTIP expected recognition period	1 year
Maximum
Summary Of Significant Accounting Policies [Line Items]
Award vesting period	5 years					4 years
Maximum | Long Term Incentive Plan
Summary Of Significant Accounting Policies [Line Items]
Unrecognized compensation expense attributable to LTIP expected recognition period	4 years
Gathering Systems | Minimum
Summary Of Significant Accounting Policies [Line Items]
Useful Lives of gathering system	20 years
Gathering Systems | Maximum
Summary Of Significant Accounting Policies [Line Items]
Useful Lives of gathering system	30 years