Long-Term Debt (Details 2) (USD $)			1 Months Ended	6 Months Ended			6 Months Ended					2 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	May 31, 2014 Line Of Credit [Member]	Jun. 30, 2014 Line Of Credit [Member]	Apr. 30, 2014 Line Of Credit [Member]	Dec. 31, 2013 Line Of Credit [Member]	Jun. 30, 2014 Line Of Credit [Member] Minimum [Member]	Jun. 30, 2014 Notes 5.875 Percent [Member]	Dec. 31, 2013 Notes 5.875 Percent [Member]	Apr. 30, 2012 Notes 5.875 Percent [Member]	Jun. 30, 2014 Notes 4.375 Percent [Member]	Jun. 30, 2014 London Interbank Offered Rate L I B O R [Member] Line Of Credit [Member]	Jun. 30, 2014 London Interbank Offered Rate L I B O R [Member] Line Of Credit [Member] Minimum [Member]	Jun. 30, 2014 London Interbank Offered Rate L I B O R [Member] Line Of Credit [Member] Maximum [Member]	Jun. 30, 2014 Debt Instrument Variable Rate Federal Funds [Member]	Jun. 30, 2014 Debt Instrument Variable Rate Federal Funds [Member] Line Of Credit [Member]	Jun. 30, 2014 Debt Instrument Variable Rate Base One Month Adjusted L I B O R [Member]	Jun. 30, 2014 Debt Instrument Variable Rate Base One Month Adjusted L I B O R [Member] Line Of Credit [Member]
Debt Instrument
Period of maturity date extension			2 years
Borrowing base of credit facility			$ 2,500,000,000		$ 2,250,000,000
Bank commitments of credit facility				1,000,000,000
Long-term debt	1,500,000,000	924,000,000				174,000,000		750,000,000	750,000,000	750,000,000	750,000,000
Letters of credit outstanding under the credit facility				2,500,000
Unused borrowing availability				$ 997,500,000
Variable rate basis												LIBOR			federal funds effective rate		adjusted one-month LIBOR
Interest rate margin (as a percent)													1.50%	2.25%		0.50%		1.00%
Additional interest rate margin, low end of range (as a percent)																0.50%		0.50%
Additional interest rate margin, high end of range (as a percent)																1.25%		1.25%
Current ratio to be maintained under credit facility							1.0
Maximum leverage ratio				3.5
Interest rate (as a percent)								5.875%	5.875%	5.875%	4.375%
Redemption price during first period (as a percent)										102.938
Redemption price after first period (as a percent)										100	100