Segment Information - Concentration Risk (Details) - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CMS [Member]
Concentration Risk [Line Items]
Net receivables from CMS	$ 1.5	$ 0.8
Government Risk Mitigation Programs Concentration Risk [Member] | CMS [Member]
Concentration Risk [Line Items]
Net receivables from CMS	$ 0.4	$ 0.3
Revenues From External Customers [Member] | Geographic Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration Risk, Benchmark Description	Foreign country
Revenues From External Customers [Member] | Geographic Concentration Risk [Member] | Maximum [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	5.00%	5.00%	5.00%
Revenue Consolidated [Member] | Government Contracts Concentration Risk [Member] | CMS [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	21.00%	21.00%	22.00%
Concentration Risk, Benchmark Description	Premiums and fees from CMS