Segment Information (Details 4) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Segment Assets
Certain cash and cash equivalents	$ 328,153	$ 345,756	$ 412,353	$ 114,332
Fair value of derivatives	20,265	30,382
Investment in unconsolidated affiliates	68,193	63,054
Total assets	8,917,716	6,728,362
Total segments
Segment Assets
Total assets	8,474,707	6,276,468
Unallocated Segment
Segment Assets
Certain cash and cash equivalents	250,133	261,473
Fair value of derivatives	20,265	30,382
Investment in unconsolidated affiliates	68,193	63,054
Other	104,418	96,985
Total assets	8,917,716	6,728,362
Marcellus | Total segments
Segment Assets
Total assets	4,143,180	3,172,144
Utica Segment | Total segments
Segment Assets
Total assets	1,379,200	439,987
Northeast Segment | Total segments
Segment Assets
Total assets	580,788	578,122
Southwest Segment | Total segments
Segment Assets
Total assets	$ 2,371,539	$ 2,086,215