Investments - Summary of Investments (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,059	$ 1,761
Gross Unrealized Gains	1	1
Gross Unrealized Losses	4	4
Estimated Fair Value	2,056	1,758
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,354	1,180
Gross Unrealized Gains	1	1
Gross Unrealized Losses	2	2
Estimated Fair Value	1,353	1,179
GSEs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	210	232
Gross Unrealized Gains	0	0
Gross Unrealized Losses	1	1
Estimated Fair Value	209	231
U.S. treasury notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	193	84
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	193	84
Municipal securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	153	143
Gross Unrealized Gains	0	0
Gross Unrealized Losses	1	1
Estimated Fair Value	152	142
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	110	69
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	110	69
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39	53
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 39	$ 53

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=73527792&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Name Accounting Standards Codification
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