Statutory Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statutory Accounting Practices [Line Items]
Statutory risk-based capital necessary to satisfy regulatory requirements	$ 3,900.0				$ 3,400.0				$ 3,900.0	$ 3,400.0
GAAP equity	23,044.1				24,251.3				23,044.1	24,251.3	$ 24,765.2	$ 23,802.7
Net income	180.9	$ 654.8	$ 859.1	$ 865.2	506.7	$ 630.9	$ 731.1	$ 701.0	2,560.0	2,569.7	2,489.7
California Department of Managed Health Care [Member]
Statutory Accounting Practices [Line Items]
Statutory risk-based capital necessary to satisfy regulatory requirements	560.0				520.0				560.0	520.0
Insurance, HMO Subsidiaries and Other Regulated Entities, Excluding the California Department of Managed Health Care [Member]
Statutory Accounting Practices [Line Items]
Statutory-basis capital and surplus	9,676.7				9,727.2				9,676.7	9,727.2
Statutory-basis net income									2,359.9	2,403.8	2,635.8
California Department of Managed Health Care Regulated Entities [Member]
Statutory Accounting Practices [Line Items]
GAAP equity	$ 1,838.1				$ 1,696.1				1,838.1	1,696.1
Net income									$ 477.5	$ 453.6	$ 487.7