Debt (Details) (USD $)						3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Credit Facility	Dec. 16, 2013 November 2020 Senior Notes	Dec. 16, 2013 September 2022 Senior Notes	Mar. 31, 2014 Low end of the range Credit Facility	Mar. 31, 2014 Low end of the range Credit Facility LIBOR	Mar. 31, 2014 Low end of the range Credit Facility Prime	Mar. 31, 2014 High end of the range Credit Facility	Mar. 31, 2014 High end of the range Credit Facility LIBOR	Mar. 31, 2014 High end of the range Credit Facility Prime	Mar. 31, 2014 Credit Facility	Dec. 31, 2013 Credit Facility	Mar. 31, 2014 Carrying amount	Dec. 31, 2013 Carrying amount	Mar. 31, 2014 Carrying amount Credit Facility		Dec. 31, 2013 Carrying amount Credit Facility		Mar. 31, 2014 Carrying amount November 2020 Senior Notes	Dec. 31, 2013 Carrying amount November 2020 Senior Notes	Mar. 31, 2014 Carrying amount September 2022 Senior Notes	Dec. 31, 2013 Carrying amount September 2022 Senior Notes	Mar. 31, 2014 Carrying amount Gross June 2014 Senior Notes	Dec. 31, 2013 Carrying amount Gross June 2014 Senior Notes	Mar. 31, 2014 Fair value	Dec. 31, 2013 Fair value
Debt obligations
Credit facility	$ 1,173,175,000	$ 1,173,175,000														$ 1,173,175,000	[1]	$ 1,173,175,000	[1]
Line of Credit Facility, Fair Value of Amount Outstanding																										1,173,175,000	1,173,175,000
Senior notes														1,122,623,000	1,127,986,000					299,970,000	300,000,000	599,163,000	600,000,000	204,936,000	205,257,000
Senior notes, net																										1,138,698,000	1,128,527,000
Net unamortized premiums														18,554,000	22,729,000
Debt, Long-term and Short-term, Combined Amount														2,295,798,000	2,301,161,000
Total debt, net				310,000,000	607,000,000																					2,311,873,000	2,301,702,000
Less current maturities														(207,502,000)	(211,558,000)
Total long-term debt, net	2,088,296,000	2,089,603,000												2,295,798,000	2,301,161,000
Credit facility variable interest rate												2.66%	2.67%
Credit facility maximum borrowing base			1,400,000,000
Lender commitments			1,200,000,000
Remaining borrowing capacity of credit facility	$ 1,000,000
Portion of properties required to maintain mortgages for credit facility	80.00%
Loans payable basis spread on variable rate							1.50%	0.50%		2.50%	1.50%
Line of credit facility, unused capacity, commitment fee (as a percent)						0.375%			0.50%

[1] Variable interest rates of 2.66% and 2.67% at March 31, 2014, and December 31, 2013, respectively.