Supplementary Quarterly Financial Information (Unaudited) - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Supplemental Quarterly Financial Information (Unaudited)
Revenues and other income	$ 121,868	$ 95,318	$ 121,813	$ 132,557	$ 178,144	$ 138,367	$ 329,166	$ 237,061	$ 471,556	$ 882,738	$ 852,153
Costs and expenses	55,903	(27,165)	171,615	185,767	(79,490)	80,776	191,875	111,309	386,120	304,470	776,199
Net income (loss)	$ 24,000	$ 60,265	$ (75,192)	$ (78,909)	$ 128,771	$ 19,123	$ 56,507	$ 74,969	$ (69,836)	$ 279,370	$ (91,044)
Net Income (Loss) Per Share
Basic (in dollars per share)	$ 0.06	$ 0.16	$ (0.20)	$ (0.21)	$ 0.34	$ 0.05	$ 0.15	$ 0.20	$ (0.18)	$ 0.73	$ (0.24)
Diluted (in dollars per share)	$ 0.06	$ 0.15	$ (0.20)	$ (0.21)	$ 0.33	$ 0.05	$ 0.15	$ 0.19	$ (0.18)	$ 0.72	$ (0.24)