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Statutory Financial Information, Capital Requirements and Restrictions on Dividends and Transfers of Funds (Statutory Financial Information) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Statutory unassigned surplus | $ 1,137.3 | $ 1,048.2 |
Statutory surplus | 1,672.9 | 1,583.8 |
Statutory net income | 221.2 | 202.3 | $ 195.8
Selective Insurace Company of America (SICA) [Member] |  |  |
Statutory unassigned surplus | 455.5 | 414.4 |
Statutory surplus | 609.7 | 568.6 |
Statutory net income | 84.6 | 72.2 | 69.6
Selective Way Insurance Company (SWIC) [Member] |  |  |
Statutory unassigned surplus | 276.1 | 260.5 |
Statutory surplus | 325.1 | 309.5 |
Statutory net income | 43.6 | 41.2 | 42.3
Selective Insurance Company of South Carolina (SICSC) [Member] |  |  |
Statutory unassigned surplus | 112.9 | 110.6 |
Statutory surplus | 144.1 | 141.9 |
Statutory net income | 17.9 | 17.4 | 15.9
Selective Insurance Company of the Southeast (SICSE) [Member] |  |  |
Statutory unassigned surplus | 86.2 | 83.5 |
Statutory surplus | 111.8 | 109.1 |
Statutory net income | 14.7 | 13.4 | 12.1
Selective Insurance Company of New York (SICNY) [Member] |  |  |
Statutory unassigned surplus | 78.8 | 74.1 |
Statutory surplus | 106.5 | 101.8 |
Statutory net income | 13.4 | 12.9 | 12.7
Selective Insurance Company of New England (SICNE) [Member] |  |  |
Statutory unassigned surplus | 16.1 | 13.6 |
Statutory surplus | 46.3 | 43.7 |
Statutory net income | 6.3 | 5.9 | 5.5
Selective Auto Insurance Company of New Jersey (SAICNJ) [Member] |  |  |
Statutory unassigned surplus | 42.1 | 36.9 |
Statutory surplus | 84.9 | 79.8 |
Statutory net income | 11.4 | 11.5 | 10.8
Mesa Underwriting Specialty Insurance Company (MUSIC) [Member] |  |  |
Statutory unassigned surplus | 21.4 | 16.7 |
Statutory surplus | 89.9 | 85.2 |
Statutory net income | 10.3 | 9.7 | 9.5
Selective Casualty Insurance Company (SCIC) [Member] |  |  |
Statutory unassigned surplus | 34.5 | 26.6 |
Statutory surplus | 109.0 | 101.0 |
Statutory net income | 13.4 | 12.6 | 12.1
Selective Fire and Casualty Insurance Company (SFCIC) [Member] |  |  |
Statutory unassigned surplus | 13.7 | 11.3 |
Statutory surplus | 45.6 | 43.2 |
Statutory net income | $ 5.6 | $ 5.5 | $ 5.3

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- Definition The amount of statutory unassigned surplus as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country. + References No definition available. + Details
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- Definition The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(23)(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24708-158529 + Details
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- Definition Amount of net income for the period determined using accounting principles prescribed or permitted by insurance regulators. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(23)(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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