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Investments - Available-for-Sale Investments (Details) $ in Millions	Jun. 30, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Debt Securities, Available-for-sale [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 1,354	$ 1,940
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses	$ 10	$ 5
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security	725	719
In a Continuous Loss Position for 12 Months or More, Estimated Fair value	$ 269	$ 227
In a Continuous Loss Position for 12 Months or More, Unrealized Losses	$ 4	$ 3
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) | Security	180	164
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 959	$ 1,297
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses	$ 7	$ 3
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security	535	561
In a Continuous Loss Position for 12 Months or More, Estimated Fair value	$ 116	$ 94
In a Continuous Loss Position for 12 Months or More, Unrealized Losses	$ 1	$ 1
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) | Security	72	69
U.S. treasury notes
Debt Securities, Available-for-sale [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 224	$ 470
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses	$ 1	$ 1
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security	52	89
In a Continuous Loss Position for 12 Months or More, Estimated Fair value	$ 0	$ 0
In a Continuous Loss Position for 12 Months or More, Unrealized Losses	$ 0	$ 0
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) | Security	0	0
GSEs
Debt Securities, Available-for-sale [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 0	$ 173
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses	$ 0	$ 1
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security	0	69
In a Continuous Loss Position for 12 Months or More, Estimated Fair value	$ 113	$ 95
In a Continuous Loss Position for 12 Months or More, Unrealized Losses	$ 2	$ 1
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) | Security	56	47
Municipal securities
Debt Securities, Available-for-sale [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value		$ 0
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses	$ 1	$ 0
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security	79	0
In a Continuous Loss Position for 12 Months or More, Estimated Fair value	$ 40	$ 38
In a Continuous Loss Position for 12 Months or More, Unrealized Losses	$ 1	$ 1
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) | Security	52	48
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 90
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses	$ 1
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security	59
In a Continuous Loss Position for 12 Months or More, Estimated Fair value	$ 0
In a Continuous Loss Position for 12 Months or More, Unrealized Losses	$ 0
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) | Security	0

X

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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Number of investment positions in available-for-sale investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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