Components of Accumulated Other Comprehensive Loss, Net - MEC (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance - Accumulated other comprehensive loss, net	$ (1,945)
Other comprehensive income (loss), unrealized gains (losses) on cash flow hedges	(8)	$ (2)
Other comprehensive income (loss)	115	68
Ending balance - Accumulated other comprehensive loss, net	(1,830)
Accumulated Other Comprehensive (Loss) Income, Net [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance - unrealized gains (losses) on available-for-sale securities	0	1,085
Beginning balance - unrealized gains (losses) on cash flow hedges	36	29
Beginning balance - Accumulated other comprehensive loss, net	(1,945)	(398)
Other comprehensive income (loss), unrealized gains on available-for-sale securities	0	0
Other comprehensive income (loss), unrealized gains (losses) on cash flow hedges	(8)	(2)
Other comprehensive income (loss)	115	68
Ending balance - unrealized gains (losses) on available-for-sale securities	0	0
Ending balance - unrealized gains (losses) on cash flow hedges	28	27
Ending balance - Accumulated other comprehensive loss, net	$ (1,830)	$ (1,415)