Unaudited Quarterly Information (Unaudited Quarterly Information) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Data [Abstract]
Revenues and other income	$ 609,204	$ 600,371	$ 601,781	$ 645,116	$ 617,257	$ 576,505	$ 601,397	$ 514,165	$ 2,456,472	$ 2,309,324	$ 1,921,791
Derivatives expense (income)	27,369	61,631	(139,109)	45,275	159,811	(210,154)	(172,904)	170,750
Other expenses	386,470	399,361	398,089	420,529	377,577	343,339	350,499	366,361	1,599,615	1,385,279	1,442,721
Net income (loss)	114,661	85,367	211,865	113,467	52,607	275,670	259,246	(14,190)	525,360	573,333	271,723
Net income (loss) per share:
Basic	$ 0.30	$ 0.22	$ 0.55	$ 0.29	$ 0.14	$ 0.69	$ 0.65	$ (0.04)	$ 1.36	$ 1.45	$ 0.73
Diluted	$ 0.30	$ 0.22	$ 0.54	$ 0.29	$ 0.13	$ 0.68	$ 0.64	$ (0.04)	$ 1.35	$ 1.43	$ 0.72
Cash flow provided by operating activities	384,765	293,506	440,966	291,654	365,722	315,739	398,521	124,832	1,410,891	1,204,814	855,811
Cash flow used for investing activities	(138,869)	(388,748)	(560,341)	(288,883)	(447,706)	(525,412)	(347,797)	(285,043)	(1,376,841)	(1,605,958)	(354,780)
Cash flow provided by (used for) financing activities	$ (118,676)	$ 91,163	$ 70,122	$ 55,902	$ 76,314	$ 112,244	$ (56,789)	$ (93,801)	$ 45,768	$ 37,968	$ (139,753)