Investments - Reconciliation of Net Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 1,501,573
Change in net unrealized investment gains on securities	$ (49,638)	$ 12,208	(209,178)	$ 79,980
Cumulative effect of change in accounting principle	0	0	(15,041)	0
Ending balance	1,304,256	1,390,441	1,304,256	1,390,441
Net Unrealized Investment Gains and Losses on Fixed Maturity and Equity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	125,596	243,510	300,177	175,738
Cumulative effect of change in accounting principle	0		(15,041)
Ending balance	75,958	255,718	75,958	255,718
Net Unrealized Investment Gains and Losses on Fixed Maturity and Equity Securities \| Fixed maturity securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	144,998	281,465	286,176	202,941
Change in net unrealized investment gains on securities	(57,903)	12,066	(186,912)	91,452
Reclassification of net investment (gains) losses to net income	(473)	2,422	2,399	1,560
Ending balance	$ 86,622	$ 295,953	$ 86,622	$ 295,953

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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