For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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=64851502&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 20
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13531-108611

Reference 3: 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=64851502&loc=d3e6404-108592

Reference 4: 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=73719730&loc=d3e13537-108611

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Ceded Reinsurance (Narrative) (Details) $ in Millions	12 Months Ended
Ceded Reinsurance (Narrative) (Details) $ in Millions	Dec. 31, 2016 USD ($) reinsurers	Dec. 31, 2015 USD ($)	Dec. 31, 2014
Concentration Risk [Line Items]
Reinsurance Recoverables, Allowance \| $	$ 1.2	$ 1.2
Reinsurance recoverable
Concentration Risk [Line Items]
Concentration risk, percentage	100.00%	100.00%
Credit concentration risk \| Premiums ceded
Concentration Risk [Line Items]
Number of ceded reinsurers \| reinsurers	4
Concentration risk, percentage	44.40%	41.10%	42.30%
Credit concentration risk \| Reinsurance recoverable \| A or better rating
Concentration Risk [Line Items]
Concentration risk, percentage	99.00%	99.00%