Summary of Significant Accounting Policies (Health Plan Services Revenue Recognition) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jan. 01, 2013
Health Care Organizations [Line Items]
Health plan services premiums	$ 15,553,348,000	$ 13,361,170,000	$ 10,377,073,000
Other assets	338,890,000	220,122,000
Other noncurrent assets	319,794,000	187,630,000
Accounts payable and other liabilities	1,874,154,000	880,374,000
Other noncurrent liabilities	$ 292,947,000	$ 242,705,000
Medicare, Medicaid and Dual Eligibles [Member]
Health Care Organizations [Line Items]
Percentage of health plan services premiums generated under contract	64.00%	59.00%	50.00%
Commercial Premium Revenue [Member]
Health Care Organizations [Line Items]
Health plan services premiums	$ 5,530,500,000	$ 5,443,100,000	$ 5,175,400,000
Commercial Premium Revenue [Member] | MLR Rebate, Premium Refund [Member]
Health Care Organizations [Line Items]
Health plan services premiums	0	0
Commercial rebate payment	400,000
Medicaid Premium Revenue [Member]
Health Care Organizations [Line Items]
Health plan services premiums	6,429,600,000	4,755,900,000	2,430,300,000
Settlement account balance, beginning				$ 0
Settlement account payment, maximum	264,000,000
State Settlement Account Surplus	386,200,000	53,400,000
Medicaid Premium Revenue [Member] | MLR Rebate, Premium Refund [Member]
Health Care Organizations [Line Items]
Health plan services premiums	(202,500,000)	(200,600,000)
Accounts payable and other liabilities	345,000,000	200,600,000
Other noncurrent liabilities	58,100,000	0
Medicaid Premium Revenue [Member] | Profit sharing, premium refund [Member]
Health Care Organizations [Line Items]
Health plan services premiums	(63,700,000)	(24,700,000)
Other noncurrent assets	0	2,300,000
Accounts payable and other liabilities	49,700,000	27,000,000
Other noncurrent liabilities	2,400,000	0
Profit Corridor Payment	36,200,000
Medicare Premium Revenue [Member]
Health Care Organizations [Line Items]
Health plan services premiums	$ 3,076,300,000	3,044,300,000	2,771,400,000
Maximum [Member] | Medicaid Premium Revenue [Member]
Health Care Organizations [Line Items]
MLR Rebate Threshold	95.00%
Minimum [Member] | Medicaid Premium Revenue [Member]
Health Care Organizations [Line Items]
MLR Rebate Threshold	85.00%
Medicare Risk adjustment [Member] | Medicare Premium Revenue [Member]
Health Care Organizations [Line Items]
Health plan services premiums	$ 1,100,000	13,100,000	(9,000,000)
Medicaid Retro Premium Adjustment [Member] | Medicaid Premium Revenue [Member]
Health Care Organizations [Line Items]
Health plan services premiums	46,900,000	0	74,300,000
State Sponsored Health Plans Settlement Account Deficit [Member] | Medicaid Premium Revenue [Member]
Health Care Organizations [Line Items]
Health plan services premiums	0	(62,900,000)	62,900,000
Other noncurrent assets	0	0	62,900,000
Other noncurrent liabilities	$ 0	$ 0
Receivable with imputed interest, discount			$ 4,400,000