Acquisitions (Details) (USD $)	9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013 Dominion Acquisition Member acre	Sep. 30, 2014 Dominion Acquisition Member	Mar. 31, 2013 Allegheny County Airport – Allegheny County Airport [Member]	Mar. 31, 2013 Allegheny County Airport – Pittsburgh International Airport [Member] [Member] acre
Business Acquisition [Line Items]
Acres Purchased			90,000,000
Business Combination, Contingent Consideration Arrangements, Change in Range of Outcomes, Contingent Consideration, Liability, Value, High			$ 190,000,000
Reimbursement From JV Partner				0.50	0.50
Payments to Acquire Property, Plant, and Equipment	1,174,607,000	1,021,127,000	91,243,000	16,000,000
Gas and Oil Area, Undeveloped, Gross						9,300
Bonus Payment in Escrow					7.60%
Bonus Payment					46,315,000
Cash Received				$ 46,311,000