Accounting Policies - Related Party Transactions (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) state	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) state	Jun. 30, 2016 USD ($)
Related Party Transaction [Line Items]
Expenses from transactions with related party	$ 8	$ 9	$ 25	$ 22
Equity Method Investee
Related Party Transaction [Line Items]
Other revenues from transactions with related party	$ 30	$ 32	$ 70	$ 70
Heartland Healthcare Services
Related Party Transaction [Line Items]
Number of states in which entity operates | state	4		4