Investments - Net Unrealized Investment Gains and Losses on Fixed Maturities and Equity Securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,501,573	$ 1,293,982
Change in unrealized investment gains (losses) on securities	(188,195)	74,405	$ 571
Cumulative effect of change in accounting principle	(15,041)	0	0
Ending balance	1,290,550	1,501,573	1,293,982
Unrealized Gains and Losses on Fixed Maturity and Equity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	300,177	175,738	175,167
Cumulative effect of change in accounting principle	(15,041)
Ending balance	96,941	300,177	175,738
Unrealized Gains and Losses on Fixed Maturity and Equity Securities \| Fixed maturity securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	286,176	202,941	201,363
Change in unrealized investment gains (losses) on securities	(172,350)	80,073	4,943
Reclassification of net investment (gains) losses on securities to net income	(12,927)	(3,162)	3,365
Ending balance	$ 111,712	$ 286,176	$ 202,941

Other Comprehensive (Income) Loss, Reclassification Adjustment From AOCI For New Accounting Pronouncements, Net Of Tax

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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.

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- Definition

Amount after tax and reclassification adjustments, of appreciation (loss) in value of unsold available-for-sale securities. Excludes amounts related to other than temporary impairment (OTTI) loss.

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- Definition

Amount after tax of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

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- Definition

Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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