Debt (Details) (USD $)	3 Months Ended		9 Months Ended			1 Months Ended	9 Months Ended				9 Months Ended						9 Months Ended						9 Months Ended					9 Months Ended					9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Oct. 31, 2011 Credit Facility [Member] Line of Credit [Member]	Sep. 30, 2011 Credit Facility [Member] Line of Credit [Member]		Dec. 31, 2010 Credit Facility [Member] Line of Credit [Member]		Sep. 30, 2011 Senior Notes Due 2017 [Member] Subordinated Debt [Member]		Sep. 30, 2009 Senior Notes Due 2017 [Member] Subordinated Debt [Member]	Mar. 10, 2011 Senior Notes Due 2017 [Member] Subordinated Debt [Member]	Dec. 31, 2010 Senior Notes Due 2017 [Member] Subordinated Debt [Member]		Sep. 30, 2011 Senior Notes Due 2018 [Member] Subordinated Debt [Member]		Sep. 30, 2008 Senior Notes Due 2018 [Member] Subordinated Debt [Member]	Mar. 10, 2011 Senior Notes Due 2018 [Member] Subordinated Debt [Member]	Dec. 31, 2010 Senior Notes Due 2018 [Member] Subordinated Debt [Member]		Sep. 30, 2010 Senior Notes Due 2020 [Member] Subordinated Debt [Member]	Sep. 30, 2011 Senior Notes Due 2020 [Member] Subordinated Debt [Member]		Dec. 31, 2010 Senior Notes Due 2020 [Member] Subordinated Debt [Member]		Sep. 30, 2010 SeniorNotesDue2021Member [Member] Subordinated Debt [Member]	Sep. 30, 2011 SeniorNotesDue2021Member [Member] Subordinated Debt [Member]		Dec. 31, 2010 SeniorNotesDue2021Member [Member] Subordinated Debt [Member]		Sep. 30, 2011 Senior Notes Due 2019 [Member] Subordinated Debt [Member]		Dec. 31, 2010 Senior Notes Due 2019 [Member] Subordinated Debt [Member]		Sep. 30, 2011 Total Debt Less Current Maturities [Member]		Dec. 31, 2010 Total Debt Less Current Maturities [Member]		Sep. 30, 2011 Less Current Maturities [Member]		Dec. 31, 2010 Less Current Maturities [Member]		Sep. 30, 2011 Debt Instruments Unamortized Discount Premium Net [Member]	Dec. 31, 2010 Debt Instruments Unamortized Discount Premium Net [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 3,119,000,000		$ 3,119,000,000		$ 2,743,000,000		$ 65,000,000		$ 0		$ 41,000,000				$ 250,000,000		$ 16,000,000				$ 256,000,000			$ 1,300,000,000		$ 1,300,000,000			$ 1,000,000,000		$ 1,000,000,000		$ 750,000,000		$ 0		$ 3,172,000,000		$ 2,806,000,000		$ 0		$ 0		$ (53,000,000)	$ (63,000,000)
Debt Instrument, Fair Value							65,000,000	[1]	0	[1]	47,000,000	[1]			288,000,000	[1]	18,000,000	[1]			279,000,000	[1]		1,334,000,000	[1]	1,396,000,000	[1]		996,000,000	[1]	1,021,000,000	[1]	687,000,000	[1]	0	[1]	3,147,000,000	[1]	2,984,000,000	[1]	0	[1]	0	[1]
Debt Instrument, Interest Rate at Period End (in hundredths)							2.92%	[2]	0.00%	[2]	12.73%	[2]			12.73%	[2]	10.25%	[2]			10.25%	[2]		9.00%	[2]	9.00%	[2]		8.00%	[2]	8.00%	[2]	6.62%	[2]	0.00%	[2]
Line of Credit Facility, Maximum Borrowing Capacity							1,500,000,000
Line of Credit Facility, redetermined borrowing base						3,000,000,000
Line Of Credit Facility Financing Fees And Expense Incurred	4,000,000		4,000,000
Line of Credit Facility, Remaining Borrowing Capacity							1,430,000,000
Outstanding letters of credit that reduce the credit facility availability							4,000,000
The percentage of properties that the company is required to maintain mortgages on (in hundredths)							80.00%
Line of credit facility, unused capacity commitment fee percentage							0.50%
Line of Credit Facility, Interest Rate Description							At the Company's election, interest on borrowings under the Credit Facility is determined by reference to either the London Interbank Offered Rate ("LIBOR") plus an applicable margin between 1.75% and 2.75% per annum (depending on the then-current level of borrowings under the Credit Facility) or the alternate base rate ("ABR") plus an applicable margin between 0.75% and 1.75% per annum (depending on the then-current level of borrowings under the Credit Facility).
Debt Instrument, Issuance Date																							April 6, 2010					September 13, 2010					May 13, 2011
Debt Instrument, Face Amount											250,000,000						256,000,000							1,300,000,000					1,000,000,000				750,000,000
Debt Instrument, Interest Rate, Stated Percentage (in hundredths)											11.75%						9.875%							8.625%					7.75%				6.50%
Debt Instrument, Maturity Date													May 15, 2017						Jul. 01, 2018				Apr. 15, 2020					Feb. 01, 2021					May 15, 2019
Issue price of debt instruments as percentage of face amount (in hundredths)																																	99.232%
Net proceeds received																																	729,000,000
Financing fees and expenses incurred related to the issuance of long-term debt																																	15,000,000
Debt Instrument, Discount																																	6,000,000
Maximum percentage of 2019 Senior Notes that can be redeemed on or before May 15, 2014																																	35.00%
Redemption price, stated as a percentage of the face amount, for 2019 Senior Notes redeemed on or before May 15, 2014																																	106.50%
Redemption price, stated as a percentage of the face amount, for 2019 Senior Notes redeemed on or after May 15, 2015, but on or before December 31, 2015																																	103.25%
Redemption price of 2019 Senior Notes if there is a change of control																																	101.00%
Debt Instrument Principal Outstanding after Redemptions														163,000,000						166,000,000
Debt Instrument Redemption Amount											87,000,000						90,000,000
Debt instrument initial tender offer total consideration											1,212.50						1,172.50
Increment amount of Senior Notes principal tendered											1,000						1,000
Consent payment included in total consideration of tender offer											30.00						30.00
Debt instrument secondary tender offer total consideration											1,182.50						1,142.50
Debt instrument total amount tendered											105,000,000						126,000,000
Debt instrument percentage tendered											65.00%						76.00%
Debt instrument repurchase amount											17,000,000						24,000,000
Debt instrument repurchase percentage											29.00%						61.00%
Loss on extinguishment of debt	$ 0	$ 0	$ 94,372,000	$ 0

[1] The carrying value of the Credit Facility is estimated to be substantially the same as its fair value. Fair values of the senior notes were estimated based on prices quoted from third-party financial institutions.

[2] Represents variable interest rate for the Credit Facility and effective interest rates for the senior notes.