Segment Information - Special Item Charges (Details) - USD ($) $ in Millions	3 Months Ended							12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
After-tax [Abstract]
Risk corridor allowance	$ 80							$ 80
Merger-related transaction costs	$ 39	$ 46	$ 26	$ 36	$ 28	$ 29		147	$ 57
Charges associated with litigation matters		$ 25						25
Debt extinguishment costs							$ 65		65
Before-tax [Abstract]
Merger-related costs, before tax								166	66
Debt extinguishment costs, before tax								0	100	$ 0
Other Operating Expenses [Member]
After-tax [Abstract]
Risk corridor allowance								80
Merger-related transaction costs								147	57
Charges associated with litigation matters								25
Debt extinguishment costs									65
Before-tax [Abstract]
Risk corridor allowance, before-tax								124
Merger-related costs, before tax								166	66
Charges associated with litigation matters, before tax								$ 40
Debt extinguishment costs, before tax									$ 100