Significant Accounting Policies (Accounting for Certain Provisions of the ACA) (Details) $ in Thousands	1 Months Ended		3 Months Ended				6 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) Customers	Mar. 31, 2015 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Jun. 30, 2015 USD ($) Customers	Jun. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2014 USD ($)
Supplementary Insurance Information, by Segment [Line Items]
Other assets			$ 501,200				$ 501,200			$ 220,122
Other noncurrent assets			300,572				300,572			187,630
Other receivables			459,090				459,090			424,910
Accounts payable and other liabilities			1,168,014				1,168,014			880,374
Other noncurrent liabilities			346,915				346,915			242,705
Health plans services premiums			4,003,432		$ 3,261,878		7,724,232	$ 6,143,223
Health Care Organization, Expenses, Net			3,363,742		2,763,179		6,506,605	5,165,521
General and administrative expenses			448,713		344,734		902,561	705,757
Administrative services fees and other income			2,712		(6,612)		3,853	(4,214)
Pretax income			132,102		98,802		199,811	160,762
ACA Health Insurer Fee [Member]
Supplementary Insurance Information, by Segment [Line Items]
ACA Health Insurer Fee Payment		$ 141,400
Deferred cost, amortization expense			58,500		37,800		116,300	74,100
Other assets			116,300				116,300			0
ACA Reinsurance Transitional Contribution [Member]
Supplementary Insurance Information, by Segment [Line Items]
General and administrative expenses			9,000		10,800		18,100	28,000
ACA Reinsurance Contribution [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums			(3,900)		(7,900)		(7,500)	(7,900)
ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
Other noncurrent assets			86,400				86,400			0
Other receivables			214,200				214,200			234,000
Reinsurance Receivable, Net			300,600	$ 273,400	110,600	$ 33,100	300,600	110,600	$ 0	234,000
ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Other noncurrent assets			34,400				34,400			0
Other receivables			53,500				53,500			81,000
Accounts payable and other liabilities			132,300				132,300			153,400
Other noncurrent liabilities			95,000				95,000			0
Risk Adjustment Payable, Net			139,400	144,500	30,100	0	139,400	30,100	0	72,400
ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
Other noncurrent assets			86,300				86,300			90,400
Other receivables			92,600				92,600			0
Other noncurrent liabilities			0				0			3,600
Risk Corridor Receivable, Net			178,900	143,400	27,300	8,700	178,900	27,300	0	$ 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			$ 1,673,100		1,121,900		$ 3,144,400	1,983,900
CALIFORNIA | Medicaid Premium Revenue [Member]
Supplementary Insurance Information, by Segment [Line Items]
Number of Customers | Customers			1,700,000				1,700,000
ARIZONA | Medicaid Premium Revenue [Member]
Supplementary Insurance Information, by Segment [Line Items]
Number of Customers | Customers			76,000				76,000
Scenario, Forecast [Member] | ACA Health Insurer Fee [Member]
Supplementary Insurance Information, by Segment [Line Items]
ACA Health Insurer Fee Payment	$ 232,600
Prior period adjustments [Member] | ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health Care Organization, Expenses, Net			$ (23,800)		0		$ (19,800)	0
Prior period adjustments [Member] | ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums			34,700		0		(6,400)	0
Prior period adjustments [Member] | ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums			(19,800)		0		5,900	0
Prior period adjustments [Member] | ACA 3Rs [Member]
Supplementary Insurance Information, by Segment [Line Items]
Pretax income			(8,900)	$ (20,300)
Current period estimates [Member] | ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health Care Organization, Expenses, Net			51,000		77,500		86,400	110,600
Current period estimates [Member] | ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums			(29,600)		(30,100)		(60,600)	(30,100)
Current period estimates [Member] | ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums			$ 55,300		$ 18,600		$ 86,200	$ 27,300