Components of Accumulated Other Comprehensive Loss, Net - MEC (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance - Accumulated other comprehensive loss, net	$ (398)
Other comprehensive income (loss), unrealized gains on available-for-sale securities	0	$ 38
Other comprehensive income (loss), unrealized gains (losses) on cash flow hedges	(2)	(4)
Other comprehensive income (loss)	68	126
Ending balance - Accumulated other comprehensive loss, net	(1,415)
Accumulated Other Comprehensive (Loss) Income, Net [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance - unrealized gains (losses) on available-for-sale securities	1,085	585
Beginning balance - unrealized gains (losses) on cash flow hedges	29	26
Beginning balance - Accumulated other comprehensive loss, net	(398)	(1,511)
Other comprehensive income (loss), unrealized gains on available-for-sale securities	0	38
Other comprehensive income (loss), unrealized gains (losses) on cash flow hedges	(2)	(4)
Other comprehensive income (loss)	68	126
Ending balance - unrealized gains (losses) on available-for-sale securities	0	623
Ending balance - unrealized gains (losses) on cash flow hedges	27	22
Ending balance - Accumulated other comprehensive loss, net	$ (1,415)	$ (1,385)