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Summary of Significant Accounting Policies - Adjustments to Amortization Expense (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Increase (decrease) to amortization expense: |  |  |
Total | $ 881 | $ (707) | $ 3,369
Retirement |  |  |
Increase (decrease) to amortization expense: |  |  |
Total | 1,081 | (313) | 3,403
Life |  |  |
Increase (decrease) to amortization expense: |  |  |
Total | $ (200) | $ (394) | $ (34)

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- Definition Amount of amortization expense (reversal of amortization expense) of deferred policy acquisition costs from a revision in estimated gross profit or estimated gross margin. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 35 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=95462302&loc=d3e10129-158409 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35755530&loc=d3e11264-158415 + Details
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