Summary of Significant Accounting Policies - Employee Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of Significant Accounting Policies - Income taxes
Employers' match of the first 6% of employee contribution (as a percent)	100.00%
Percentage of eligible compensation, matched 100% by employer	6.00%
Basic matching contributions recorded	$ 61	$ 10	$ 2

Amount of cost for defined contribution plan.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration