Investments - Summary of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,730	$ 1,808
Gross Unrealized Gains	5	0
Gross Unrealized Losses	0	7
Estimated Fair Value	1,735	1,801
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,123	1,189
Gross Unrealized Gains	4	0
Gross Unrealized Losses	0	5
Estimated Fair Value	1,127	1,184
GSEs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	259	212
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	1
Estimated Fair Value	259	211
Municipal securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	147	186
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	1
Estimated Fair Value	148	185
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	76	63
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	76	63
U.S. treasury notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	65	78
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	65	78
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	60	80
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 60	$ 80

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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