Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Schedule of Premium Revenue by Health Plan Type [Line Items]
Number of categories for component of premium revenue	2
Maximum period for member risk scores and member pharmacy cost experience after original year of service	2 years
Anticipated Medicare risk adjustment premiums	$ 33.4	$ 20.8
California, Florida, Illinois, New Mexico, Washington
Schedule of Premium Revenue by Health Plan Type [Line Items]
Liability recorded related to profit sharing agreement	2.0	1.4
New Mexico,Texas, and Washington
Schedule of Premium Revenue by Health Plan Type [Line Items]
Liability recorded related to profit sharing agreement	$ 11.1	$ 2.5
Illinois, New Mexico, Ohio, Texas, Washington, and Wisconsin | Minimum
Schedule of Premium Revenue by Health Plan Type [Line Items]
Percentage of incremental revenue earned maximum	1.00%
Illinois, New Mexico, Ohio, Texas, Washington, and Wisconsin | Maximum
Schedule of Premium Revenue by Health Plan Type [Line Items]
Percentage of incremental revenue earned maximum	5.00%