Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Tax benefit on net other comprehensive loss recognized during the period for postretirement benefit plans	$ 375	$ 668	$ 192
Tax expense on amounts reclassified from accumulated other comprehensive loss during the period for postretirement benefit plans	$ 437	$ 382	$ 464