Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)		12 Months Ended
	Jul. 02, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Significant Accounting Policies [Line Items]
Maximum dividend allowed prior to merger	$ 0.29
Member co-share amounts and government subsidies		$ 12,300,000,000	$ 9,700,000,000	$ 7,300,000,000
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%
Benefits expense		$ 44,269,000,000	38,166,000,000	32,564,000,000
Document Fiscal Year Focus		2015
New Tricare South Region Contract
Significant Accounting Policies [Line Items]
TRICARE Health care cost payments		$ 3,300,000,000	3,200,000	3,200,000,000
TRICARE Health care cost reimbursements		3,300,000,000	3,200,000,000	3,200,000
Difference between TRICARE health care costs payments and reimbursements		$ (4,000,000)		5,000,000
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
Commercial Medical
Significant Accounting Policies [Line Items]
Percentage of premiums and services revenue		18.00%
CMS Reinsurance Subsidies Or Discounts
Significant Accounting Policies [Line Items]
Subsidy and discount payments		$ 8,900,000,000	6,700,000,000	4,800,000,000
Subsidy and discount reimbursements		8,600,000,000	5,800,000,000	4,600,000,000
Subsidy and discount payments in excess of reimbursements		$ (361,000,000)	(945,000,000)	(154,000,000)
Settlement period after close		9 months
CMS Brand Name Prescription Drug Discounts [Member] | Minimum
Significant Accounting Policies [Line Items]
Settlement period after close		14 months
CMS Brand Name Prescription Drug Discounts [Member] | Maximum
Significant Accounting Policies [Line Items]
Settlement period after close		18 months
Agreement and Plan of Merger With Aetna Inc.
Significant Accounting Policies [Line Items]
Number of shares one share of common stock is converted into	0.8375
Cash received for each share converted	$ 125
Total transaction cost, including assumption of debt	$ 37,000,000,000
MCCI
Significant Accounting Policies [Line Items]
Related party receivable		$ 284,000,000
Interest rate on receivable		10.00%
Maximum borrowing capacity		$ 55,000,000
Advances to affiliate		9,000,000
Benefits expense		1,037,000,000	962,000,000	873,000,000
Health and Human Services | Reinsurance Contribution
Significant Accounting Policies [Line Items]
Member co-share amounts and government subsidies		478,000,000	281,000,000	0
Government and health plan member payments		(409,000,000)	(255,000,000)	0
Government and health plan member subsidies, amount in excess of payments		$ 69,000,000	$ 26,000,000	$ 0