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Statutory Financial Information, Capital Requirements, and Restrictions on Dividends and Transfers of Funds (Dividends) (Details) - USD ($) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Total stockholders’ equity | $ 1,712,957,000 | $ 1,531,370,000 | $ 1,398,041,000
Ordinary dividends paid | 80,100,000 |  |
Maximum ordinary dividends | $ 211,000,000 |  |
Maximum borrowing percentage | 10.00% |  |
New Jersey Dividend Restriction | 10.00% |  |
New York Dividend Restriction | 10.00% |  |
Dividend Restriction New York | 100.00% |  |
Selective Insurace Company of America (SICA) [Member] |  |  |
Ordinary dividends paid | $ 28,000,000 |  |
Maximum ordinary dividends | 77,600,000 |  |
Selective Way Insurance Company (SWIC) [Member] |  |  |
Ordinary dividends paid | 19,000,000 |  |
Maximum ordinary dividends | 43,300,000 |  |
Selective Insurance Company of South Carolina (SICSC) [Member] |  |  |
Ordinary dividends paid | 10,000,000 |  |
Maximum ordinary dividends | 17,900,000 |  |
Selective Insurance Company of the Southeast (SICSE) [Member] |  |  |
Ordinary dividends paid | 7,500,000 |  |
Maximum ordinary dividends | 14,700,000 |  |
Selective Insurance Company of New York (SICNY) [Member] |  |  |
Ordinary dividends paid | 4,500,000 |  |
Maximum ordinary dividends | 10,700,000 |  |
Selective Insurance Company of New England (SICNE) [Member] |  |  |
Ordinary dividends paid | 2,000,000 |  |
Maximum ordinary dividends | 6,200,000 |  |
Selective Auto Insurance Company of New Jersey (SAICNJ) [Member] |  |  |
Ordinary dividends paid | 2,500,000 |  |
Maximum ordinary dividends | 11,300,000 |  |
Mesa Underwriting Specialty Insurance Company (MUSIC) [Member] |  |  |
Ordinary Dividends Paid To Parent | 2,100,000 |  |
Maximum ordinary dividends | 10,300,000 |  |
Selective Casualty Insurance Company (SCIC) [Member] |  |  |
Ordinary dividends paid | 3,000,000 |  |
Maximum ordinary dividends | 13,400,000 |  |
Selective Fire and Casualty Insurance Company (SFCIC) [Member] |  |  |
Ordinary dividends paid | 1,500,000 |  |
Maximum ordinary dividends | 5,600,000 |  |
Parent Company [Member] |  |  |
Total stockholders’ equity | 1,700,000,000 |  |
Maximum ordinary dividends | 114,500,000 |  |
Additional Borrowing Capacity for Parent | $ 70,500,000 |  |

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