Debt - Revolving Credit and Letter of Credit (Details) (USD $)	12 Months Ended			1 Months Ended	12 Months Ended							12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Revolving Credit And Letter Of Credit Facility [Member]	Dec. 31, 2013 Revolving Credit And Letter Of Credit Facility [Member] Banks	Dec. 31, 2012 Revolving Credit And Letter Of Credit Facility [Member] Minimum [Member]	Dec. 31, 2012 Revolving Credit And Letter Of Credit Facility [Member] Maximum [Member]	Dec. 31, 2013 Letter of Credit [Member]	Dec. 31, 2012 Letter of Credit [Member]	Dec. 31, 2013 Primary Commitment Holders [Member] Revolving Credit And Letter Of Credit Facility [Member] Banks	Dec. 31, 2013 Minor Commitment Holders [Member] Revolving Credit And Letter Of Credit Facility [Member] Banks	Dec. 31, 2012 HealthSpring [Member]	Jan. 31, 2012 HealthSpring [Member]
Business Acquisition [Line Items]
Debt													$ 326,000,000
Repayment of long-term debt	15,000,000	326,000,000	451,000,000									326,000,000
Line of Credit Facility [Line Items]
Line of Credit Facility, Initiation Date					Dec. 01, 2012
Line of Credit Facility, Expiration Date					Dec. 01, 2017
Line of Credit Facility, Term of Agreement				5 years
Line of Credit Facility, Maximum Borrowing Capacity				1,500,000,000					500,000,000
Line of Credit Facility, Number of Participating Banks					16					3	13
Line of Credit Facility, Collective Bank Percentage Committment										35.00%	65.00%
Line of Credit Facility, Optional Maximum Borrowing Capacity				2,000,000,000
Line of Credit Facility, Covenant Terms					This agreement has certain covenants, including a financial covenant requiring the Company to maintain a total debt-to-adjusted capital ratio at or below 0.50 to 1.00
Maximum debt-to-adjusted capital ratio convenant						50.00%	100.00%
Credit Facility Remaining Borrowing Capacity					6,000,000,000
Short-term and Long-term Debt Outstanding	5,200,000,000
Letters of Credit Outstanding, Amount								$ 39,000,000