Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended			9 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012	Sep. 30, 2014
Significant Accounting Policies [Line Items]
Typical term (in years) of short-duration employer-group prepaid health services policies	1 year
Number of days notice for employer group to cancel short-duration prepaid health services policies	30 days
Consumer discounts on brand name prescription drugs	50.00%
Member co-share amounts and government subsidies	$ 9,700,000,000	$ 7,300,000,000	$ 6,300,000,000
New Tricare South Region Contract
Significant Accounting Policies [Line Items]
TRICARE Health care cost payments	3,200,000,000	3,200,000,000		2,100,000,000
TRICARE Health care cost reimbursements	3,200,000,000	3,200,000,000		2,000,000,000
Difference between TRICARE health care costs payments and reimbursements		5,000,000		56,000,000
Equipment | Minimum
Significant Accounting Policies [Line Items]
Useful life	3 years
Equipment | Maximum
Significant Accounting Policies [Line Items]
Useful life	10 years
Computer Software | Minimum
Significant Accounting Policies [Line Items]
Useful life	3 years
Computer Software | Maximum
Significant Accounting Policies [Line Items]
Useful life	7 years
Building | Minimum
Significant Accounting Policies [Line Items]
Useful life	20 years
Building | Maximum
Significant Accounting Policies [Line Items]
Useful life	40 years
Health Care Reform
Significant Accounting Policies [Line Items]
Payment of annual health insurance industry fee					562,000,000
Commercial Medical
Significant Accounting Policies [Line Items]
Percentage of premiums and services revenue	18.00%
Reinsurance Contribution
Significant Accounting Policies [Line Items]
Member co-share amounts and government subsidies	281,000,000
Payments to HHS	255,000,000
Member co-share amounts and government subsidies in excess of payments	26,000,000
CMS Reinsurance Subsidies Or Discounts
Significant Accounting Policies [Line Items]
Subsidy And Discount Payments	6,700,000,000	4,800,000,000
Subsidy And Discount Reimbursements	5,800,000,000	4,600,000,000
Subsidy And Discount Payments In Excess Of Reimbursements	$ 945,000,000	$ 154,000,000