Significant Accounting Policies (Accounting for Certain Provisions of the ACA) (Details) (USD $)	3 Months Ended		1 Months Ended	12 Months Ended	3 Months Ended	1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Sep. 30, 2014	Dec. 31, 2013	Jun. 30, 2014	Sep. 30, 2015	Dec. 31, 2014
Supplementary Insurance Information, by Segment [Line Items]
Other assets	$ 348,240,000						$ 220,122,000
Other noncurrent assets	288,954,000						187,630,000
Other receivables	367,135,000						424,910,000
Accounts payable and other liabilities	1,180,454,000						880,374,000
Other noncurrent liabilities	290,714,000						242,705,000
Health plans services premiums	3,720,800,000	2,881,345,000
Health Care Organization, Expenses, Net	3,142,863,000	2,402,342,000
General and administrative expenses	453,848,000	361,023,000
Administrative services fees and other income	1,141,000	2,398,000
Pretax income	67,709,000	61,960,000
ACA Health Insurer Fee [Member]
Supplementary Insurance Information, by Segment [Line Items]
ACA Health Insurer Fee Payment			141,400,000
Deferred cost, amortization expense	57,800,000	36,300,000
Other assets	173,500,000						0
ACA Reinsurance Transitional Contribution [Member]
Supplementary Insurance Information, by Segment [Line Items]
General and administrative expenses	9,100,000	17,200,000
ACA Reinsurance Contribution [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums	(3,600,000)	0
ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
Other noncurrent assets	35,400,000						0
Other receivables	238,000,000						234,000,000
Reinsurance Receivable, Net	273,400,000						234,000,000
Health Care Organization, Expenses, Net	(39,400,000)	(33,100,000)
ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Other noncurrent assets	15,000,000						0
Other receivables	36,500,000						81,000,000
Accounts payable and other liabilities	150,000,000						153,400,000
Other noncurrent liabilities	46,000,000						0
Risk Adjustment Payable, Net	(144,500,000)						(72,400,000)
ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
Other noncurrent assets	143,400,000						90,400,000
Other noncurrent liabilities	0						3,600,000
Risk Corridor Receivable, Net	143,400,000						86,800,000
No Risk [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums		0		4,400,000
Health Care Organization, Expenses, Net		0		0
General and administrative expenses		0		4,400,000
Administrative services fees and other income		1,400,000		6,500,000
Pretax income		1,400,000		6,500,000
Full Risk [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums					154,700,000
Health Care Organization, Expenses, Net					144,000,000
General and administrative expenses					0
Administrative services fees and other income					(7,900,000)
Pretax income					2,800,000
Medicaid Premium Revenue [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plans services premiums	1,471,300,000	862,000,000
CALIFORNIA | Medicaid Premium Revenue [Member]
Supplementary Insurance Information, by Segment [Line Items]
Number of Customers	1,600,000
ARIZONA | Medicaid Premium Revenue [Member]
Supplementary Insurance Information, by Segment [Line Items]
Number of Customers	81,000
Scenario, Forecast [Member] | ACA Health Insurer Fee [Member]
Supplementary Insurance Information, by Segment [Line Items]
ACA Health Insurer Fee Payment						231,300,000
Prior period adjustments [Member] | ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
Reinsurance Receivable, Net	4,000,000
Prior period adjustments [Member] | ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Risk Adjustment Payable, Net	(41,100,000)
Prior period adjustments [Member] | ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
Risk Corridor Receivable, Net	25,700,000
Prior period adjustments [Member] | ACA 3Rs [Member]
Supplementary Insurance Information, by Segment [Line Items]
Pretax income	11,400,000
Current period estimates [Member] | ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
Reinsurance Receivable, Net	35,400,000
Current period estimates [Member] | ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Risk Adjustment Payable, Net	(31,000,000)
Current period estimates [Member] | ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
Risk Corridor Receivable, Net	30,900,000
Total period estimate [Member] | ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
Reinsurance Receivable, Net	39,400,000
Total period estimate [Member] | ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Risk Adjustment Payable, Net	(72,100,000)
Total period estimate [Member] | ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
Risk Corridor Receivable, Net	$ 56,600,000