Long-Term Debt (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Nov. 17, 2014	May 20, 2013	Jun. 29, 2012	Nov. 08, 2011	Feb. 07, 2011
Debt Instrument [Line Items]
Senior Notes	$ 2,200,000,000	$ 1,600,000,000
Debt Conversion, Converted Instrument, Amount			2,641,000
Interest Costs, Capitalized During Period	16,200,000	11,000,000	12,100,000
Senior Notes [Member]
Debt Instrument [Line Items]
6.625% Senior Notes, Due 2019 Face Value	350,000,000	350,000,000
6.50% Senior Notes, Due 2021 Face Value	350,000,000	350,000,000
6.125% Senior Notes, Due 2022	600,000,000	0
6.50% Senior Notes, Due 2023 Face Value	400,000,000	400,000,000
5% Senior Notes, Due 2024 Face Value	500,000,000	500,000,000
Senior Notes	2,200,000,000	1,600,000,000
Debt Instrument, Covenant Compliance, Dividends Excluded From Computation	6,500,000
Senior Notes [Member] | 6.125% Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount					600,000,000
Debt Instrument, Interest Rate, Stated Percentage					6.125%
Net proceeds from debt issuance	590,000,000
Deferred Finance Costs, Gross					10,000,000
Maximum percentage of the principal amount of the debt instrument which the entity may redeem with proceeds from equity offerings in the first through third year following the issuance year (as a percent)	35.00%
Redemption price of debt instrument if redeemed with proceeds from qualified equity offerings in the first through third year following the issuance year (as a percent)	106.125%
Debt Instrument, Call Feature	at a redemption price equal to 100 percent of the principal amount of the 2022 Notes to be redeemed, plus a specified make-whole premium and accrued and unpaid interest to the applicable redemption date.
Maximum Penalty as a Percentage of Face Value for Not Registering Senior Notes Within One Year of Issuance					1.00%
Senior Notes [Member] | 5% Senior Notes Due 2024 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount						500,000,000
Debt Instrument, Interest Rate, Stated Percentage						5.00%
Net proceeds from debt issuance		490,200,000
Deferred Finance Costs, Gross						9,800,000
Redemption price of debt instrument if redeemed with proceeds from qualified equity offerings in the first through third year following the issuance year (as a percent)	105.00%
Debt Instrument, Call Feature	at a redemption price equal to 100 percent of the principal amount of the 2024 Notes to be redeemed, plus a specified make-whole premium and accrued and unpaid interest to the applicable redemption date.
Senior Notes [Member] | 6.50% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount							400,000,000
Debt Instrument, Interest Rate, Stated Percentage							6.50%
Net proceeds from debt issuance			392,100,000
Deferred Finance Costs, Gross							7,900,000
Maximum percentage of the principal amount of the debt instrument which the entity may redeem with proceeds from equity offerings in the first through third year following the issuance year (as a percent)	35.00%
Redemption price of debt instrument if redeemed with proceeds from qualified equity offerings in the first through third year following the issuance year (as a percent)	106.50%
Debt Instrument, Call Feature	redemption price equal to 100 percent of the principal amount of the 2023 Notes to be redeemed, plus a specified make-whole premium and accrued and unpaid interest to the applicable redemption date.
Senior Notes [Member] | 6.50% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount								350,000,000
Debt Instrument, Interest Rate, Stated Percentage								6.50%
Net proceeds from debt issuance				343,100,000
Deferred Finance Costs, Gross								6,900,000
Debt Instrument, Call Feature	redemption price equal to 100 percent of the principal amount, plus a specified make-whole premium and accrued and unpaid interest
Senior Notes [Member] | 6.625% Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount									350,000,000
Debt Instrument, Interest Rate, Stated Percentage									6.625%
Net proceeds from debt issuance				341,100,000
Deferred Finance Costs, Gross									8,900,000
Senior Notes [Member] | 2015 [Member] | 6.625% Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	103.313%
Senior Notes [Member] | 2016 [Member] | 6.50% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	103.25%
Senior Notes [Member] | 2016 [Member] | 6.625% Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	101.656%
Senior Notes [Member] | 2017 [Member] | 6.50% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	103.25%
Senior Notes [Member] | 2017 [Member] | 6.50% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	102.167%
Senior Notes [Member] | 2018 [Member] | 6.125% Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	103.063%
Senior Notes [Member] | 2018 [Member] | 5% Senior Notes Due 2024 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	102.50%
Senior Notes [Member] | 2018 [Member] | 6.50% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	102.167%
Senior Notes [Member] | 2018 [Member] | 6.50% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	101.083%
Senior Notes [Member] | 2019 [Member] | 6.125% Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	101.531%
Senior Notes [Member] | 2019 [Member] | 5% Senior Notes Due 2024 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	101.677%
Senior Notes [Member] | 2019 [Member] | 6.50% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	101.083%
Senior Notes [Member] | 2020 [Member] | 5% Senior Notes Due 2024 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	100.833%
Senior Notes [Member] | 2020 and Thereafter [Member] | 6.125% Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	100.00%
Senior Notes [Member] | 2020 and Thereafter [Member] | 6.50% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	100.00%
Senior Notes [Member] | 2019 and Thereafter [Member] | 6.50% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	100.00%
Senior Notes [Member] | 2017 and Thereafter [Member] | 6.625% Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	100.00%
Senior Notes [Member] | 2021 and Thereafter [Member] | 5% Senior Notes Due 2024 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	100.00%
Convertible Debt [Member] | 3.50% Senior Convertible Notes
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			3.50%
Conversion of 3.50% Senior Convertible Notes to common stock, including income tax benefit of conversion (shares)			864,106
Debt Conversion, Converted Instrument, Amount			281,300,000
Repayments of Convertible Debt			$ 6,200,000