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Investments OTTI Credit Losses Recognized in Earnings (Detail) - Fixed maturities - USD ($) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward] |  |  |
Beginning balance | $ 11,500,000 | $ 12,800,000 | $ 13,100,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment | 400,000 | 0 | 0
Reductions for securities sold/matured | (11,200,000) | 0 | (1,400,000)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows | (200,000) | (1,300,000) | 1,100,000
Ending balance | 500,000 | 11,500,000 | 12,800,000
Residential mortgage-backed securities |  |  |
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward] |  |  |
Beginning balance | 11,100,000 | 12,400,000 | 12,700,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment | 0 | 0 | 0
Reductions for securities sold/matured | (10,900,000) | 0 | (1,400,000)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows | (200,000) | (1,300,000) | 1,100,000
Ending balance | 0 | 11,100,000 | 12,400,000
Commercial mortgage-backed securities |  |  |
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward] |  |  |
Beginning balance | 400,000 | 400,000 | 400,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment | 400,000 | 0 | 0
Reductions for securities sold/matured | (300,000) | 0 | 0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows | 0 | 0 | 0
Ending balance | $ 500,000 | $ 400,000 | $ 400,000

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- Definition Current period changes in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held due to changes in the cash flows expected to be collected and recognized over the remaining lives of the securities. + References No definition available. + Details
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- Definition This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which no such other than temporary impairment (OTTI) was previously recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563 + Details
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- Definition The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563 + Details
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- Definition This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security because of the sale of impaired securities during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563 + Details
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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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