Commitments and Contingencies - Broker credit risk (Details) - Gross written premium - Property and casualty operations - Credit concentration risk	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Concentration Risk [Line Items]
Maximum percentage of ceded written premium	10.00%
AON Corporation and subsidiaries
Concentration Risk [Line Items]
Maximum percentage of ceded written premium	17.00%	18.00%	18.00%
Marsh & McLennan Companies
Concentration Risk [Line Items]
Maximum percentage of ceded written premium	18.00%	18.00%	19.00%
Willis Group and subsidiaries
Concentration Risk [Line Items]
Maximum percentage of ceded written premium	10.00%	11.00%	13.00%

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=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 20
-URI http://asc.fasb.org/extlink&oid=77997519&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=84177426&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=77997519&loc=d3e13537-108611

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