Components of Accumulated Other Comprehensive Loss, Net - MEC (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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	$ 81	0	$ 119
Other comprehensive income (loss), unrealized gains (losses) on cash flow hedges	3	(2)	1	(6)
Other comprehensive income (loss)	(250)	296	(182)	422
Ending balance - Accumulated other comprehensive loss, net	(1,665)		(1,665)
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	119
Other comprehensive income (loss), unrealized gains (losses) on cash flow hedges			1	(6)
Other comprehensive income (loss)			(182)	422
Ending balance - unrealized gains (losses) on available-for-sale securities	0	704	0	704
Ending balance - unrealized gains (losses) on cash flow hedges	30	20	30	20
Ending balance - Accumulated other comprehensive loss, net	$ (1,665)	$ (1,089)	$ (1,665)	$ (1,089)