Basis of Presentation - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 1,290,550
Other comprehensive income (loss) before reclassifications	$ (107,163)	$ 52,873	(227,466)	$ 162,412
Amounts reclassified from AOCI	(114,925)	429	(121,330)	2,872
Net current period other comprehensive income (loss)	(7,762)	(52,444)	106,136	(174,581)
Cumulative effect of change in accounting principle	0	0	0	(15,041)
Ending balance	1,499,677	1,350,441	1,499,677	1,350,441
Other income tax expense (benefit)	30,549	(115)	32,252	(764)
Total [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	198,654	164,823	84,756	286,960
Cumulative effect of change in accounting principle	0	0	0	(15,041)
Ending balance	190,892	112,379	190,892	112,379
Net Unrealized Investment Gains (Losses) on Securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	210,839	178,040	96,941	300,177
Other comprehensive income (loss) before reclassifications	(107,163)	52,873	(227,466)	162,412
Amounts reclassified from AOCI	(114,925)	429	(121,330)	2,872
Net current period other comprehensive income (loss)	(7,762)	(52,444)	106,136	(174,581)
Cumulative effect of change in accounting principle				(15,041)
Ending balance	203,077	125,596	203,077	125,596
Pretax reclassification amounts from accumulated other comprehensive income (loss)	145,474	(544)	153,582	(3,636)
Net Funded Status of Benefit Plans [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(12,185)	(13,217)	(12,185)	(13,217)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from AOCI	0	0	0	0
Net current period other comprehensive income (loss)	0	0	0	0
Cumulative effect of change in accounting principle				0
Ending balance	$ (12,185)	$ (13,217)	$ (12,185)	$ (13,217)

Other Comprehensive Income (Loss), Net Of Tax, Adjusted

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

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

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Amount before 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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Amount of other income tax expense (benefit).

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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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