Significant Accounting Policies (Accounting for Certain Provisions of the ACA) (Details) $ in Thousands	1 Months Ended			3 Months Ended				9 Months Ended		12 Months Ended
	Oct. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) Customers	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) Customers	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Sep. 30, 2015 USD ($) Customers	Sep. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Supplementary Insurance Information, by Segment [Line Items]
Other assets		$ 309,543		$ 309,543				$ 309,543				$ 220,122
Other noncurrent assets		270,097		270,097				270,097				187,630
Other receivables		430,987		430,987				430,987				424,910
Accounts payable and other liabilities		1,486,133		1,486,133				1,486,133				880,374
Other noncurrent liabilities		277,922		277,922				277,922				242,705
Health plans services premiums				3,977,891	$ 3,631,617			11,702,123	$ 9,774,840
Health Care Organization, Expenses, Net				3,318,415	3,104,010			9,825,020	8,269,531
General and administrative expenses				442,326	373,623			1,344,887	1,079,380
Administrative services fees and other income				1,200	1,106			5,053	(3,108)
Pretax income				153,348	22,723			353,159	183,485
ACA Health Insurer Fee [Member]
Supplementary Insurance Information, by Segment [Line Items]
ACA Health Insurer Fee Payment		233,000	$ 141,400
Deferred cost, amortization expense				58,400	31,900			174,700	106,100
Other assets		58,200		58,200				58,200				0
ACA Reinsurance Transitional Contribution [Member]
Supplementary Insurance Information, by Segment [Line Items]
General and administrative expenses				9,000	13,700			27,100	41,700
ACA Reinsurance Contribution [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums				(3,700)	(5,900)			(11,300)	(13,800)
ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
Other noncurrent assets		0		0				0				0
Other receivables		167,800		167,800				167,800				234,000
Reinsurance Receivable, Net		167,800	142,600	167,800	142,600	$ 110,600		167,800	142,600	$ 0	$ 300,600	234,000
ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Other noncurrent assets		0		0				0				0
Other receivables		123,200		123,200				123,200				81,000
Accounts payable and other liabilities		144,900		144,900				144,900				153,400
Other noncurrent liabilities		0		0				0				0
Risk Adjustment Payable, Net		21,700	47,700	21,700	47,700	(30,100)		21,700	47,700	0	(139,400)	72,400
ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
Other noncurrent assets		176,900		176,900				176,900				90,400
Other receivables		12,400		12,400				12,400				0
Other noncurrent liabilities		0		0				0				3,600
Risk Corridor Receivable, Net		$ 189,300	$ 72,000	189,300	72,000	27,300		189,300	72,000	0	$ 178,900	$ 86,800
No Risk [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums							$ 0			4,400
Health Care Organization, Expenses, Net							0			0
General and administrative expenses							0			4,400
Administrative services fees and other income							1,400			6,500
Pretax income							$ 1,400			$ 6,500
Full Risk [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums						154,700
Health Care Organization, Expenses, Net						144,000
General and administrative expenses						0
Administrative services fees and other income						(7,900)
Pretax income						$ 2,800
Medicaid Premium Revenue [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums				$ 1,668,100	1,397,700			$ 4,812,500	3,381,600
CALIFORNIA | Medicaid Premium Revenue [Member]
Supplementary Insurance Information, by Segment [Line Items]
Number of Customers | Customers		1,800,000		1,800,000				1,800,000
ARIZONA | Medicaid Premium Revenue [Member]
Supplementary Insurance Information, by Segment [Line Items]
Number of Customers | Customers		67,000		67,000				67,000
Scenario, Forecast [Member] | ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
2014 Risk corridor payment expected in 2015	$ 12,400
2014 Risk corridor payment expected in 2015_percentage	12.60%
Prior period adjustments [Member] | ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health Care Organization, Expenses, Net				$ 0	0			$ (19,800)	0
Prior period adjustments [Member] | ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums				200	0			(6,200)	0
Prior period adjustments [Member] | ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums				5,500	0			11,400	0
Prior period adjustments [Member] | ACA 3Rs [Member]
Supplementary Insurance Information, by Segment [Line Items]
Pretax income				5,700				(14,600)
Current period estimates [Member] | ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health Care Organization, Expenses, Net				65,700	32,000			152,100	142,600
Current period estimates [Member] | ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums				28,400	(17,600)			(32,200)	(47,700)
Current period estimates [Member] | ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums				4,900	44,700			91,100	72,000
Payments Made (Received) [Member] | ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
ACA Reinsurance recovery payment received				(198,500)	0			(198,500)	0
Payments Made (Received) [Member] | ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
ACA Risk adjustment payment				89,100	0			89,100	0
Payments Made (Received) [Member] | ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
ACA Risk corridor payment received				$ 0	$ 0			$ 0	$ 0