Components of Accumulated Other Comprehensive Loss, Net - MEC (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 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	$ 423	0	$ 542
Other comprehensive income (loss), unrealized gains (losses) on cash flow hedges	1	1	2	(5)
Other comprehensive income (loss)	(2)	666	(184)	1,088
Ending balance - Accumulated other comprehensive loss, net	(1,667)		(1,667)
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	542
Other comprehensive income (loss), unrealized gains (losses) on cash flow hedges			2	(5)
Other comprehensive income (loss)			(184)	1,088
Ending balance - unrealized gains (losses) on available-for-sale securities	0	1,127	0	1,127
Ending balance - unrealized gains (losses) on cash flow hedges	31	21	31	21
Ending balance - Accumulated other comprehensive loss, net	$ (1,667)	$ (423)	$ (1,667)	$ (423)