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Summary of Significant Accounting Policies - New Accounting Pronouncements, Adopted (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restricted cash and cash equivalents		$ 8			$ 17	$ 6	$ 78
Cumulative-effect of adopting new accounting guidance				$ 15
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect of adopting new accounting guidance				$ 15
ASU 2014-09 | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect of adopting new accounting guidance			$ 16
Scenario, Forecast | ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating leases, right-of-use assets	$ 800
Operating leases, liability	$ 800

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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Present value of lessee's discounted obligation for lease payments from operating lease.

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