Investments (Short-Term and Long-Term Investments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 18,205	$ 19,331
Securities, Available for sale Debt Securities, Gross Unrealized Gains	387	317
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(41)	(221)
Available-for-sale Securities, Debt Securities	18,551	19,427
Available-for-sale Equity Securities, Amortized Cost Basis	1,511	1,576
Securities, Available for sale Equity Securities, Gross Unrealized Gains	36	9
Securities, Available for sale Equity Securities, Gross Unrealized Losses	(25)	(13)
Available-for-sale Securities, Equity Securities	1,522	1,572
Held-to-maturity securities, Amortized Cost	495us-gaap_HeldToMaturitySecurities	543us-gaap_HeldToMaturitySecurities
Securities, Held to maturity, Unrecognized Holding Gain	2	1
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-maturity Securities, Fair Value	497	544
Total investments, Amortized Cost	20,211unh_InvestmentsAmortizedCost	21,450unh_InvestmentsAmortizedCost
Total investments, Gross Unrealized Gains	425unh_InvestmentsGrossUnrealizedGains	327unh_InvestmentsGrossUnrealizedGains
Total investments, Gross Unrealized Losses	(66)unh_InvestmentsGrossUnrealizedLosses	(234)unh_InvestmentsGrossUnrealizedLosses
Investments, Fair Value Disclosure	20,570us-gaap_InvestmentsFairValueDisclosure	21,543us-gaap_InvestmentsFairValueDisclosure
Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	18,551	19,427
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,614	2,211
Securities, Available for sale Debt Securities, Gross Unrealized Gains	7	5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(1)	(21)
Available-for-sale Securities, Debt Securities	1,620	2,195
Held-to-maturity securities, Amortized Cost	178	181
Securities, Held to maturity, Unrecognized Holding Gain	2	1
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-maturity Securities, Fair Value	180	182
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	6,456	6,902
Securities, Available for sale Debt Securities, Gross Unrealized Gains	217	147
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(5)	(72)
Available-for-sale Securities, Debt Securities	6,668	6,977
Held-to-maturity securities, Amortized Cost	19	28
Securities, Held to maturity, Unrecognized Holding Gain	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-maturity Securities, Fair Value	19	28
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	7,241	7,265
Securities, Available for sale Debt Securities, Gross Unrealized Gains	112	130
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(26)	(60)
Available-for-sale Securities, Debt Securities	7,327	7,335
Held-to-maturity securities, Amortized Cost	298	334
Securities, Held to maturity, Unrecognized Holding Gain	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-maturity Securities, Fair Value	298	334
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,022	2,256
Securities, Available for sale Debt Securities, Gross Unrealized Gains	39	23
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(5)	(61)
Available-for-sale Securities, Debt Securities	2,056	2,218
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	872	697
Securities, Available for sale Debt Securities, Gross Unrealized Gains	12	12
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(4)	(7)
Available-for-sale Securities, Debt Securities	$ 880	$ 702

Investments at amortized cost.

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