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Fair Value Measurements - Transfers between Level 1 and Level 2 (Details) $ in Millions	Dec. 31, 2017 USD ($) stock	Dec. 31, 2016 USD ($) stock	Dec. 31, 2015 USD ($) stock
Perpetual preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of stocks transferred from Level 2 to Level 1 | stock	4	6	5
Fair value of assets transferred from Level 2 to Level 1 | $	$ 23	$ 35	$ 19
Number of stocks transferred from Level 1 to Level 2 | stock	2	7	7
Fair value of assets transferred from Level 1 to Level 2 | $	$ 11	$ 28	$ 31
Common stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of stocks transferred from Level 2 to Level 1 | stock	0	3	7
Fair value of assets transferred from Level 2 to Level 1 | $	$ 0	$ 0	$ 80
Number of stocks transferred from Level 1 to Level 2 | stock	1	2	0
Fair value of assets transferred from Level 1 to Level 2 | $	$ 0	$ 0	$ 0
Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of stocks transferred from Level 2 to Level 1 | stock	0	0	2
Fair value of assets transferred from Level 2 to Level 1 | $	$ 0	$ 0	$ 11
Number of stocks transferred from Level 1 to Level 2 | stock	0	0	0
Fair value of assets transferred from Level 1 to Level 2 | $	$ 0	$ 0	$ 0

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Number of stocks transferred from Level 1 to Level 2.

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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

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ 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

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