Investments - Realized Gains and Losses and Proceeds on Investments and Other Assets (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Long-Term Investments:
Realized Investment Gains (Losses)			$ 9
Gross Proceeds from Sale/Maturity			21
Loss on debt conversions
Loss on debt redemptions		$ (2)	(6)
Gross Proceeds from Sale/Maturity		0	0
Property, Plant and Equipment
Net Realized Gains (Losses)			2
Gross Proceeds from Sale/Maturity	$ 21	0	4
Investments in unconsolidated affiliates
Net Realized Gains (Losses)				$ (3)
Gross Proceeds from Sale/Maturity				0
Other intangible assets
Net Realized Gains (Losses)	5		(1)	(1)
Gross Proceeds from Sale/Maturity	21		0	0
Other Assets:
Net Realized Gains (Losses)		(2)	(1)	(3)	$ 11
Gross Proceeds from Sale/Maturity		0	0	6	0
Total:
Net Realized Gains (Losses)	1	(4)	2	(8)	11
Gross Proceeds from Sale/Maturity	319	236	1,003	689	1,185
Debt Securities
Available-for-sale Securities:
Gross Realized Gains	3	3	7	4	14
Gross Realized Losses	0	(3)	(8)	(16)	(17)
Net Realized Gains (Losses)	3	0	(1)	(12)	(3)
Gross Proceeds from Sale/Maturity	298	$ 236	968	624	1,076
Preferred stock available for sale
Available-for-sale Securities:
Gross Realized Gains			0	1	1
Gross Realized Losses			0	0	0
Net Realized Gains (Losses)	(3)		0	1	1
Gross Proceeds from Sale/Maturity	0		$ 10	9	58
Equity securities available for sale
Available-for-sale Securities:
Gross Realized Gains				11	13
Gross Realized Losses				(1)	(11)
Net Realized Gains (Losses)	(4)			10	2
Gross Proceeds from Sale/Maturity	$ 0			$ 50	$ 51

Gain (Loss) on Other Assets [Abstract]

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Investments in Unconsolidated Affiliates [Abstract]

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Loss on Debt Conversions and Debt Refinancing [Abstract]

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Other Intangible Assets [Abstract]

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Proceeds from Debt Conversions and Debt Refinancing

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This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
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This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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This item represents the gain (loss) realized during the period from the sale of investments accounted for under the cost method of accounting and any losses recognized thereon for impairments of other than a temporary nature.

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-Publisher FASB
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Amount of gain (loss) on sale or disposal of intangible assets.

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Amount of gain (loss) on disposal of other assets utilized in financial service operations.

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Amount of gain (loss) on sale or disposal of property, plant and equipment assets, including oil and gas property and timber property.

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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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-Publisher FASB
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The cash inflow associated with the sale or maturity (principal being due) of securities not classified as either held-to-maturity securities or trading securities which are classified as available-for-sale securities.

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The cash inflow associated with the sale and maturity (principal being due) of other investments, prepayment and call (request of early payment) of other investments not otherwise defined in the taxonomy.

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The cash inflow associated with the sale, maturity and collection of all investments such as debt, security and so forth during the period.

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The cash inflow associated with the sale of equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

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The cash inflow from disposal of asset without physical form usually arising from contractual or other legal rights, excluding goodwill.

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Amount of cash inflow from the sale of other assets as part of operating activities.

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The cash inflow from the sale of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale.

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This item represents the total realized gain (loss) included in earnings for the period as a result of selling marketable securities categorized as trading, available-for-sale, or held-to-maturity and any gains or losses realized during the period from the sale of investments accounted for under the cost method of accounting and/or investments that are separately or otherwise not categorized as trading, available-for-sale, or held-to-maturity. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.

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The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

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