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Available-for-sale securities - Summary of available-for-sale debt and equity securities at estimated fair value (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | $ 34,756 | $ 30,325
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 282 | 81
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | (102) | (115)
Available-for-sale Securities | 34,936 | 30,291
Corporate debt securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 14,790 | 12,657
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 3 | 7
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | (46) | (61)
Available-for-sale Securities | 14,747 | 12,603
Certificates of deposit |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 5,131 | 943
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Available-for-sale Securities | 5,131 | 943
Money market funds |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 4,714 | 5,464
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Available-for-sale Securities | 4,714 | 5,464
U.S. treasury securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 4,090 | 5,558
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 0 | 1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | (29) | (30)
Available-for-sale Securities | 4,061 | 5,529
Residential mortgage-backed and asset-backed securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 4,072 | 3,613
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 1 | 2
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | (15) | (13)
Available-for-sale Securities | 4,058 | 3,602
U.S. government agencies securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 934 | 981
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | (8) | (6)
Available-for-sale Securities | 926 | 975
Non-U.S. government securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 668 | 725
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | (4) | (5)
Available-for-sale Securities | 664 | 720
Municipal debt securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 0 | 27
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Available-for-sale Securities | 0 | 27
Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 357 | 357
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 278 | 71
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Available-for-sale Securities | $ 635 | $ 428

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562 + Details
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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563 + Details
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- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563 + Details
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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References 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 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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