Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Non-service related postretirement costs		$ 87	$ 118	$ 118
Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Tax reform, reclassification from AOCI to retained earnings, tax effect	$ 163
Reclassifying Adjustment | Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Non-service related postretirement costs			$ 118	$ 118
Subsequent Event | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use assets					$ 450
Operating lease, liabilities					$ 575