Investments - Available-for-Sale Investments (Details) $ in Millions	Dec. 31, 2015 USD ($) Security	Dec. 31, 2014 USD ($) Security
Schedule of Available-for-sale Securities [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 1,296	$ 540
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses	$ 6	$ 1
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) \| Security	1,143	416
In a Continuous Loss Position for 12 Months or More, Estimated Fair value	$ 124	$ 43
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	99	26
Corporate Debt Securities, GSEs, and Municipal Securities
Schedule of Available-for-sale Securities [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 1,135
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses	$ 6
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) \| Security	846
In a Continuous Loss Position for 12 Months or More, Estimated Fair value	$ 119
In a Continuous Loss Position for 12 Months or More, Unrealized Losses	$ 1
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) \| Security	87
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 825	$ 379
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses	$ 4	$ 1
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) \| Security	588	265
In a Continuous Loss Position for 12 Months or More, Estimated Fair value	$ 119	$ 29
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	87	10
GSEs
Schedule of Available-for-sale Securities [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 182	$ 75
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	77	22
In a Continuous Loss Position for 12 Months or More, Estimated Fair value	$ 0	$ 3
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	3
Municipal securities
Schedule of Available-for-sale Securities [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 128	$ 54
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	181	64
In a Continuous Loss Position for 12 Months or More, Estimated Fair value	$ 5	$ 11
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	12	13
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 53	$ 13
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses	$ 0	$ 0
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) \| Security	218	52
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
U.S. treasury notes
Schedule of Available-for-sale Securities [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 53	$ 19
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses	$ 0	$ 0
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) \| Security	32	13
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
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 55
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses	$ 0
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) \| Security	47
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

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

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