Statutory Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statutory Accounting Practices [Line Items]
Statutory risk-based capital necessary to satisfy regulatory requirements	$ 4,300.0				$ 3,900.0				$ 4,300.0	$ 3,900.0
GAAP equity	25,100.4				23,044.1				25,100.4	23,044.1	$ 24,251.3	$ 24,765.2
Net income	368.4	$ 617.8	$ 780.6	$ 703.0	180.9	$ 654.8	$ 859.1	$ 865.2	2,469.8	2,560.0	2,569.7
California Department of Managed Health Care [Member]
Statutory Accounting Practices [Line Items]
Statutory risk-based capital necessary to satisfy regulatory requirements	650.0				560.0				650.0	560.0
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	10,580.2				9,676.7				10,580.2	9,676.7
Statutory-basis net income									2,613.2	2,359.9	2,403.8
California Department of Managed Health Care Regulated Entities [Member]
Statutory Accounting Practices [Line Items]
GAAP equity	$ 2,225.7				$ 1,838.1				2,225.7	1,838.1
Net income									$ 774.5	$ 477.5	$ 453.6