Summary of Significant Accounting Policies (Accounting for Certain Provisions of the ACA) (Details) $ in Thousands	1 Months Ended		3 Months Ended								12 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2015 USD ($) Customers	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) Customers	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Supplementary Insurance Information, by Segment [Line Items]
Other assets			$ 338,890				$ 220,122				$ 338,890	$ 220,122
Other receivables			467,526				424,910				467,526	424,910
Accounts payable and other liabilities			1,874,154				880,374				1,874,154	880,374
Other noncurrent assets			319,794				187,630				319,794	187,630
Other noncurrent liabilities			$ 292,947				242,705				$ 292,947	242,705
Number of customers | Customers			6,100,000								6,100,000
Health plan services premiums											$ 15,553,348	13,361,170	$ 10,377,073
Health plan services expenses			$ 3,216,000	$ 3,318,400	$ 3,363,700	$ 3,142,900	3,038,200	$ 3,104,000	$ 2,763,200	$ 2,402,300	13,041,036	11,307,751	8,886,547
General and administrative expenses											1,816,166	1,552,364	1,083,694
Administrative services fees and other income											6,294	(1,725)	34,791
ACA Health Insurer Fee [Member]
Supplementary Insurance Information, by Segment [Line Items]
ACA Health Insurer Fee Payment	$ 233,000	$ 141,400									233,000	141,400
Amortization of deferred cost			58,200	$ 58,400	$ 58,500	$ 57,800	35,400	$ 31,900	37,800	36,300	233,000	141,400
Other assets			0				0				0	0
ACA Reinsurance Transitional Contribution [Member]
Supplementary Insurance Information, by Segment [Line Items]
General and administrative expenses											36,000	55,000
ACA Reinsurance Contribution [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plan services premiums											(14,300)	(15,900)
ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
Other receivables			214,100				234,000				214,100	234,000
Reinsurance Receivable, Net			214,100				234,000				214,100	234,000	0
ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Other receivables			143,200				81,000				143,200	81,000
Accounts payable and other liabilities			187,200				153,400				187,200	153,400
Risk Adjustment Payable, Net			44,000				72,400				44,000	72,400	0
ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
Other receivables			1,800				0				1,800	0
Other noncurrent assets			212,500				90,400				212,500	90,400
Other noncurrent liabilities			0				3,600				$ 0	3,600
2014 Risk corridor payment received in 2015_percentage											12.60%
Risk Corridor Receivable, Net			$ 214,300				$ 86,800				$ 214,300	86,800	0
No Risk [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plan services premiums										0			4,400
Health plan services expenses										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 plan services premiums									154,700
Health plan services expenses									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 plan services premiums											$ 6,429,600	4,755,900	$ 2,430,300
Medicaid Premium Revenue [Member] | CALIFORNIA
Supplementary Insurance Information, by Segment [Line Items]
Number of customers | Customers			1,800,000								1,800,000
Medicaid Premium Revenue [Member] | ARIZONA
Supplementary Insurance Information, by Segment [Line Items]
Number of customers | Customers			65,000								65,000
Prior period adjustments [Member] | ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plan services expenses											$ (19,300)	0
Prior period adjustments [Member] | ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plan services premiums											(6,300)	0
Prior period adjustments [Member] | ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plan services premiums											11,300	0
Prior period adjustments [Member] | ACA 3Rs [Member]
Supplementary Insurance Information, by Segment [Line Items]
Pretax income											(14,300)
Current period estimates [Member] | ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plan services expenses											214,100	234,000
Current period estimates [Member] | ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plan services premiums											(49,600)	(72,400)
Current period estimates [Member] | ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plan services premiums											126,800	86,800
Payments Made (Received) [Member] | ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
ACA Reinsurance recovery payment received											(214,700)	0
Payments Made (Received) [Member] | ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
ACA Risk adjustment payment											84,300	0
Payments Made (Received) [Member] | ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
ACA Risk corridor payment received											$ (10,600)	$ 0