Fair Value Measurements (Details) (USD $)	3 Months Ended			3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012 7.35% Senior Notes due 2017 [Member]	Mar. 31, 2012 7.5% Senior Notes due 2018 [Member]	Dec. 31, 2011 7.5% Senior Notes due 2018 [Member]	Mar. 31, 2012 7.15% Senior Notes due 2018 [Member]	Mar. 31, 2012 4.10% Senior Notes Due 2022 [Member]	Mar. 31, 2012 Carrying Amount [Member]	Dec. 31, 2011 Carrying Amount [Member]	Mar. 31, 2012 Fair Value [Member]	Dec. 31, 2011 Fair Value [Member]	Mar. 31, 2012 Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]	Dec. 31, 2011 Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]	Mar. 31, 2012 Significant Observable Inputs, Level 2 [Member]	Dec. 31, 2011 Significant Observable Inputs, Level 2 [Member]	Mar. 31, 2012 Significant Unobservable Inputs, Level 3 [Member]	Dec. 31, 2011 Significant Unobservable Inputs, Level 3 [Member]
Cash and cash equivalents								$ 206,167,000	$ 15,627,000	$ 206,167,000	$ 15,627,000
Unsecured revolving credit facility								0	671,500,000	0	671,500,000
Senior notes								1,670,580,000	671,800,000	1,802,255,000	773,578,000
Debt instrument, interest rate, stated percentage	7.15%		7.35%	7.50%		7.15%	4.10%
Debt instrument maturity				2018
Yield, Publicly-traded debt market				3.50%	4.60%		4.20%
Derivative assets										834,329,000	717,343,000	0	0	649,413,000	534,560,000	184,916,000	182,783,000
Derivative liabilities										(14,714,000)	(12,513,000)	0	0	(14,341,000)	(11,849,000)	(373,000)	(664,000)
Total derivative instruments, net								819,615,000	704,830,000	819,615,000	704,830,000	0	0	635,072,000	522,711,000	184,543,000	182,119,000
Balance at beginning of period	182,119,000	97,677,000
Total gains or losses (realized/unrealized): Included in earnings	58,462,000	15,910,000
Total gains or losses (realized/unrealized): Included in other comprehensive income (loss)	290,000	(15,306,000)
Purchases, issuances, and settlements:
Purchases	0	0
Issuances	0	0
Settlements	(56,328,000)	(12,843,000)
Transfers into/out of Level 3	0	142,000
Balance at end of period	184,543,000	85,580,000
Change in unrealized gain (loss) included in earnings relating to derivatives still held as of March 31	$ 2,134,000	$ 3,067,000