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Fair Value Measurements - Investments Held in Trusts (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 | Dec. 31, 2018
Debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost Basis | $ 736 | $ 714
Unrealized Gains | 27 | 2
Unrealized Losses | (14) | (28)
Fair Value | 749 | 688
Equity securities |  |
Amortized cost basis, unrealized gains and losses and fair values of investments in available-for-sale securities |  |
Cost Basis | 312 | 339
Unrealized Gains | 50 | 15
Unrealized Losses | (2) | (16)
Fair Value | $ 360 | $ 338

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 + Details
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