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Fair value measurements Fair Value Measurements (Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Balance- Beginning of year, assets |  | $ 0 |
Transfers into Level 3, assets |  | 0 |
Balance- End of year, assets |  |  | $ 0
Other Long-term Investments [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Balance- Beginning of year, assets | $ 225 | 212 | 204
Transfers into Level 3, assets | 0 |  |
Change in Net Unrealized Gains (Losses) included in OCI, Assets | 6 | (2) | (6)
Net Realized Gains/Losses, Assets | 0 | 1 |
Purchased, assets | 56 | 33 | 33
Sales, assets | 0 |  |
Settlements, assets | (24) | (19) | (19)
Balance- End of year, assets | 263 | 225 | 212
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets | 0 | 1 | 0
Short-term investments |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Balance- Beginning of year, assets | 25 |  |
Transfers into Level 3, assets | 0 | 0 |
Transfers out of Level 3, assets |  | (50) |
Change in Net Unrealized Gains (Losses) included in OCI, Assets | 0 | 0 |
Net Realized Gains/Losses, Assets | 0 | 0 |
Purchased, assets | 16 | 75 |
Sales, assets | 0 | 0 |
Settlements, assets | (41) | 0 |
Balance- End of year, assets |  | 25 |
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets | 0 | 0 |
Equity Securities [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Balance- Beginning of year, assets | 41 | 16 | 2
Transfers into Level 3, assets | 0 |  |
Change in Net Unrealized Gains (Losses) included in OCI, Assets | (1) | 2 | 3
Net Realized Gains/Losses, Assets | 2 | 1 | (2)
Purchased, assets | 24 | 27 | 13
Sales, assets | (22) | (5) |
Settlements, assets | 0 |  |
Balance- End of year, assets | 44 | 41 | 16
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets | (1) |  | (2)
Collateralized Mortgage Backed Securities [Member] | Available-for-sale Securities [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Balance- Beginning of year, assets | 45 | 53 | 15
Transfers into Level 3, assets | 50 | 0 |
Change in Net Unrealized Gains (Losses) included in OCI, Assets | 0 | (1) |
Net Realized Gains/Losses, Assets | 0 | 0 |
Purchased, assets | 8 | 1 | 41
Sales, assets | (1) | (8) | (2)
Settlements, assets | (24) |  | (1)
Balance- End of year, assets | 78 | 45 | 53
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets | 0 | 0 | 0
Corporate securities | Available-for-sale Securities [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Balance- Beginning of year, assets | 681 | 174 | 187
Transfers into Level 3, assets | 231 | 53 | 16
Transfers out of Level 3, assets | (93) | (10) |
Change in Net Unrealized Gains (Losses) included in OCI, Assets | (12) | 15 | (1)
Net Realized Gains/Losses, Assets | 0 | (13) | (4)
Purchased, assets | 521 | 566 | 52
Sales, assets | (111) | (59) | (28)
Settlements, assets | (180) | (45) | (48)
Balance- End of year, assets | 1,037 | 681 | 174
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets | (2) | (11) | (2)
Foreign Government Debt Securities [Member] | Available-for-sale Securities [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Balance- Beginning of year, assets | 74 | 57 | 22
Transfers into Level 3, assets | 0 | 9 | 34
Transfers out of Level 3, assets | (3) | (24) |
Change in Net Unrealized Gains (Losses) included in OCI, Assets | 3 | 1 | (2)
Net Realized Gains/Losses, Assets | 0 | (6) | (1)
Purchased, assets | 84 | 70 | 15
Sales, assets | (59) | (17) | (3)
Settlements, assets | (6) | (16) | (8)
Balance- End of year, assets | 93 | 74 | 57
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets | $ (1) | $ (5) | $ (1)

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- Definition Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3). + 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 (d) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Amount of gain (loss) recognized in the income statement for 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)(1) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Amount of gain (loss) recognized in other comprehensive income (loss) for 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)(1a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Amount of purchases 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)(2) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Amount of sale 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)(2) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Amount of settlement 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)(2) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy. + 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)(3) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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