Significant Accounting Policies (Accounting for Certain Provisions of the ACA) (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Supplementary Insurance Information, by Segment [Line Items]
Accounts payable and other liabilities	$ 669,406,000		$ 397,036,000
Other assets	254,702,000		132,683,000
Other noncurrent assets	190,550,000		162,551,000
Health plan services (excluding depreciation and amortization)	2,402,342,000	2,268,736,000
Health plan services premiums	2,881,345,000	2,632,069,000
ACA Health Insurer Fee [Member]
Supplementary Insurance Information, by Segment [Line Items]
Accounts payable and other liabilities	145,000,000
Deferred cost, amortization expense	36,300,000
Other assets	108,900,000
ACA Reinsurance Recovery [Member]
Supplementary Insurance Information, by Segment [Line Items]
Other noncurrent assets	33,100,000
Health plan services (excluding depreciation and amortization)	(33,100,000)
ACA Risk Adjustment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Health plan services premiums	0
ACA Risk Corridor [Member]
Supplementary Insurance Information, by Segment [Line Items]
Other noncurrent assets	8,700,000
Health plan services premiums	$ 8,700,000