Debt - Maturities (Details) - USD ($)	Dec. 31, 2017	Jun. 06, 2017
Maturities of Long-term Debt [Abstract]
Net carrying amount	$ 2,041,000,000
2018	0
2019	0
2020	550,000,000
2021	0
2022	1,000,000,000
Thereafter	$ 491,000,000
5.375% Notes
Maturities of Long-term Debt [Abstract]
Percentage of contractual interest rate	5.375%
1.125% Convertible Notes
Maturities of Long-term Debt [Abstract]
Percentage of contractual interest rate	1.125%
4.875% Notes
Maturities of Long-term Debt [Abstract]
Percentage of contractual interest rate	4.875%
1.625% Convertible Notes
Maturities of Long-term Debt [Abstract]
Percentage of contractual interest rate	1.625%
Senior Notes \| 5.375% Notes
Maturities of Long-term Debt [Abstract]
Net carrying amount	$ 700,000,000
2018	0
2019	0
2020	0
2021	0
2022	700,000,000
Thereafter	$ 0
Percentage of contractual interest rate	5.375%
Senior Notes \| 4.875% Notes
Maturities of Long-term Debt [Abstract]
Net carrying amount	$ 330,000,000
2018	0
2019	0
2020	0
2021	0
2022	0
Thereafter	$ 330,000,000
Percentage of contractual interest rate	4.875%	4.875%
Convertible Debt \| 1.125% Convertible Notes
Maturities of Long-term Debt [Abstract]
Net carrying amount	$ 550,000,000
2018	0
2019	0
2020	550,000,000
2021	0
2022	0
Thereafter	$ 0
Percentage of contractual interest rate	1.125%
Convertible Debt \| 1.625% Convertible Notes
Maturities of Long-term Debt [Abstract]
Net carrying amount	$ 161,000,000
2018	0
2019	0
2020	0
2021	0
2022	0
Thereafter	$ 161,000,000
Percentage of contractual interest rate	1.625%
Credit Facility
Maturities of Long-term Debt [Abstract]
Net carrying amount	$ 300,000,000
2018	0
2019	0
2020	0
2021	0
2022	300,000,000
Thereafter	$ 0

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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Contractual interest rate for funds borrowed, under the debt agreement.

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- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04.(a))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04.(a))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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