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Nature of Business and Summary of Significant Accounting Policies (Other Comprehensive Income) - Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cost of sales |  |  |  |  |  |  |  |  | $ (279,541) | $ (270,695) | $ (293,843)
Defined benefit plan adjustments – actuarial losses |  |  |  |  |  |  |  |  | (1,294) | 2,628 |
Income before provision for income taxes |  |  |  |  |  |  |  |  | 44,687 | 46,895 | 25,708
Tax (expense) benefit |  |  |  |  |  |  |  |  | (20,847) | (11,666) | (7,062)
Total reclassifications for the period, net of tax | $ (11,110) | $ 15,898 | $ 12,401 | $ 6,651 | $ 7,572 | $ 12,415 | $ 10,228 | $ 5,014 | 23,840 | 35,229 | 18,646
Reclassification Out of Accumulated Other Comprehensive Income [Member] |  |  |  |  |  |  |  |  |  |  |
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |  |  |  |  |  |  |  |
Defined benefit plan adjustments – actuarial losses |  |  |  |  |  |  |  |  | (406) | (156) | (396)
Income before provision for income taxes |  |  |  |  |  |  |  |  | 3,381 | 4,833 | 9,749
Tax (expense) benefit |  |  |  |  |  |  |  |  | (1,423) | (1,897) | (3,834)
Total reclassifications for the period, net of tax |  |  |  |  |  |  |  |  | 1,958 | 2,936 | 5,915
Unrealized Gains (Losses) on Available-for-Sale Securities [Member] | Reclassification Out of Accumulated Other Comprehensive Income [Member] |  |  |  |  |  |  |  |  |  |  |
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net realized investment gain |  |  |  |  |  |  |  |  | 4,864 | 5,408 | 10,348
Impairment Expense [Member] | Reclassification Out of Accumulated Other Comprehensive Income [Member] |  |  |  |  |  |  |  |  |  |  |
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net realized investment gain |  |  |  |  |  |  |  |  | (180) | $ (419) | $ (203)
Net Losses on Derivatives Designated as Hedging Instruments [Member] | Reclassification Out of Accumulated Other Comprehensive Income [Member] |  |  |  |  |  |  |  |  |  |  |
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cost of sales |  |  |  |  |  |  |  |  | $ (897) |  |

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- Definition Total costs related to goods produced and sold during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2(a)) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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- Definition Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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