Segment Information - Concentration Risk (Details) - Revenue Consolidated [Member] - Customer Concentration Risk [Member]	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Concentration Risk [Line Items]
Concentration Risk, Percentage	18.00%	17.00%
Concentration Risk, Benchmark Description	Consolidated revenues
Concentration Risk, Additional Characteristic	Premiums and fees from CMS

For an entity that discloses a concentration risk as a percentage of some financial balance or benchmark, this item identifies a second characteristic of the concentration. For example, "Europe" is the second characteristic in the phrase: "net assets in Europe are 70% of total net assets".

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 954
-SubTopic 310
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=68075217&loc=d3e3115-115594

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592

Name:	us-gaap_ConcentrationRiskAdditionalCharacteristic
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration