Accumulated Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Pre-Tax Amount
December 31, 2017	$ 12,044
Pretax Cumulative Effect of New Accounting Principal in Period of Adoption		$ (12,300)
Changes in net unrealized gains on investments	(18,407)
AOCI Before Tax Attributable to Parent after Cumulative Effect Adjustment		(256)
Reclassification adjustment for realized gains	674
September 30, 2018	(17,989)
Tax (Expense) Benefit
December 31, 2017	(2,823)
Tax Cumulative Effect of New Accounting Principal in Period of Adoption		2,962
AOCI Tax Attributable to Parent After Cumulative Effect Adjustment		139
Changes in net unrealized gains on investments	4,316
Reclassification adjustment for realized gains	(168)
September 30, 2018	4,287
Cumulative Effect of New Accounting Principle in Period of Adoption	9,338	(9,338)
Net-of-Tax Amount
December 31, 2017	9,221
AOCI Net of Tax After Cumulative Effect Adjustment		$ (117)
Changes in net unrealized gains on investments	(14,091)
Reclassification adjustment for realized gains	(506)
September 30, 2018	$ (13,702)

AOCI Before Tax Attributable to Parent after Cumulative Effect Adjustment

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AOCI Net of Tax After Cumulative Effect Adjustment

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AOCI Tax Attributable to Parent After Cumulative Effect Adjustment

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Pre-Tax Amount [Abstract]

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Pretax Cumulative Effect of New Accounting Principal in Period of Adoption

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Tax Cumulative Effect of New Accounting Principal in Period of Adoption

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Tax (Expense) Benefit [Abstract]

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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Amount before tax of increase (decrease) in accumulated equity from transactions and other events and circumstances from non-owner sources, attributable to parent. Excludes net income (loss), and accumulated changes in equity from transactions resulting from investments by owners (distributions to owners).

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Amount of tax expense (benefit) allocated to accumulated other comprehensive income (loss) attributable to parent.

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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Amount before tax and reclassification adjustments of other comprehensive income (loss) attributable to parent.

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Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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Amount of tax expense (benefit) allocated to other comprehensive income (loss) before reclassification adjustment from accumulated other comprehensive income (loss).

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Amount before tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

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