Segment Reporting (Total Assets for Reportable Segments) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Assets	$ 4,814,723	$ 5,245,267
Western [Member]
Segment Reporting Information [Line Items]
Assets	1,492,989	1,482,474
Central [Member]
Segment Reporting Information [Line Items]
Assets	1,219,958	1,187,505
Eastern [Member]
Segment Reporting Information [Line Items]
Assets	866,421	852,963
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Assets	1,123,733	1,609,553
Corporate [Member]
Segment Reporting Information [Line Items]
Assets	$ 111,622	$ 112,772