Income Taxes (Schedule of Reconciliation between the Statutory Federal Income Tax Rate and the Effective Tax Rate) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Differences between the statutory federal income tax rate and the effective tax rate
Provision for income taxes at statutory federal rate, Amount	$ (991)	$ (600)	$ 144
State income taxes, net of federal income tax benefit, Amount	(10)	(1)	13
Net income attributable to noncontrolling interests, Amount	(22)	(33)	(35)
Change in valuation allowance, Amount	26	(1)	(2)
Federal rate change, Amount	32
Federal and state tax credits, Amount	(5)	(6)	(5)
Nondeductible transaction costs, Amount		3
Nondeductible goodwill	504	536
Other, Amount	17	(2)	1
Total (benefit from) provision for income taxes for (loss) income from continuing operations	$ (449)	$ (104)	$ 116
Provision for income taxes at statutory federal rate, Percentage	35.00%	35.00%	35.00%
State income taxes, net of federal income tax benefit, Percentage	0.30%	0.10%	3.30%
Net income attributable to noncontrolling interests, Percentage	0.80%	1.90%	(8.60%)
Change in valuation allowance, Percentage	(0.90%)	0.10%	(0.40%)
Federal rate change, Percentage	(1.10%)
Federal and state tax credits, Percentage	0.10%	0.30%	(1.20%)
Nondeductible transaction costs, Percentage		(0.20%)
Nondeductible goodwill, Percentage	(17.80%)	(31.20%)
Other, Percentage	(0.60%)	0.10%	0.30%
Total (benefit from) provision for income taxes for (loss) income from continuing operations, Percentage	15.80%	6.10%	28.40%