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Fair Value - Additional Information (Detail) - USD ($) | Mar. 31, 2016 | Dec. 31, 2015 | Mar. 31, 2015 | Dec. 31, 2014
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Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Transfers between Level 1 and Level 2 | $ 0 |  | $ 0 |
Level 3 assets fair value | $ 9,000,000 | $ 11,000,000 | $ 12,000,000 | $ 32,000,000
Upper limit percentage of Level 3 assets to total invested assets | 0.10% |  |  |
Short-term borrowings | $ 300,000,000 | 299,000,000 |  |
Carrying Value |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Long-term debt, fair value | 3,793,000,000 | 3,794,000,000 |  |
Fair Value |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Long-term debt, fair value | 4,035,000,000 | 3,986,000,000 |  |
Commercial Paper |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Short-term borrowings | $ 300,000,000 | $ 299,000,000 |  |

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- Definition Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13476-108611 + Details
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- Definition Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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