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Debt - Schedule of Long-Term Debt (Detail) - USD ($) $ in Millions | Jun. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Debt Instrument [Line Items] |  |
Long-term debt | $ 6,796 | $ 8,048
Less: amount due within one year |  | 4
Long-term debt, excluding amount due within one year | 6,796 | 8,044
Long-term debt | 6,796 | 8,048
6.50% Bonds, Due 2018 [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt |  | 250
Long-term debt |  | 250
6.75% Notes, Due 2018 [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt |  | 750
Long-term debt |  | 750
5.72% Notes, Due 2019 [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt | 500 | 750
Long-term debt | 500 | 750
1.63% Convertible Notes, Due 2019 [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt | 403 | 403
Long-term debt | 403 | 403
6.150% Notes, Due 2020 [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt | 1,000 | 1,000
Long-term debt | 1,000 | 1,000
5.40% Notes, Due 2021 [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt | 1,250 | 1,250
Long-term debt | 1,250 | 1,250
5.87% Notes, Due 2022 [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt | 627 | 627
Long-term debt | 627 | 627
5.125% Notes, Due 2024 [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt | 1,250 | 1,250
Long-term debt | 1,250 | 1,250
5.90% Notes, Due 2027 [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt | 625 | 625
Long-term debt | 625 | 625
6.75% Bonds, Due 2028 [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt | 300 | 300
Long-term debt | 300 | 300
5.95% Notes Due 2037 [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt | 625 | 625
Long-term debt | 625 | 625
Iowa Finance Authority Loan, Due 2042 [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt | 250 | 250
Long-term debt | 250 | 250
Other [Member] |  |
Debt Instrument [Line Items] |  |
Long-term debt | 34 | 32
Long-term debt | $ 34 | $ 32

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- Definition Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: 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(a)(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: 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 Reference 3: 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 + Details
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