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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13531-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13537-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

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

Contracts Accounted for as Insurance - Narrative (Details)	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Guarantor Obligations [Line Items]
Weighted average risk-free rates for U.S. dollar denominated financial guaranty insurance obligations	3.04%		2.39%
Minimum [Member]
Guarantor Obligations [Line Items]
Weighted average risk-free rates for U.S. dollar denominated financial guaranty insurance obligations	0.00%		0.00%
Maximum [Member]
Guarantor Obligations [Line Items]
Weighted average risk-free rates for U.S. dollar denominated financial guaranty insurance obligations	3.20%		2.78%
Foreign Currency Concentration Risk [Member] \| Premiums Receivable [Member]
Guarantor Obligations [Line Items]
Percentage of installment premiums denominated in currencies other than the U.S. dollar	72.00%	71.00%	72.00%