Investments - Reconciliation of Net Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 1,501,573
Change in net unrealized investment gains on securities	$ (52,444)	$ 45,239	(159,540)	$ 67,772
Cumulative effect of change in accounting principle	0	0	(15,041)	0
Ending balance	1,350,441	1,362,130	1,350,441	1,362,130
Net Unrealized Investment Gains and Losses on Fixed Maturity and Equity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	178,040	198,271	300,177	175,738
Cumulative effect of change in accounting principle	0		(15,041)
Ending balance	125,596	243,510	125,596	243,510
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	206,293	228,337	286,176	202,941
Change in net unrealized investment gains on securities	(61,724)	54,193	(129,009)	79,386
Reclassification of net investment (gains) losses to net income	429	(1,065)	2,872	(862)
Ending balance	$ 144,998	$ 281,465	$ 144,998	$ 281,465

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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