Fair Value (Activity Under Level 3, Fair Value Measurements Using Significant Unobservable Inputs By Asset Type) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers in and/or (out) of Level 3	$ (3,818)	$ 3,977	$ (3,818)	$ 1,519
Market Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	881,063	374,476	702,331	383,994
Total gains or (losses) (realized/unrealized), Included in earnings	(224)	(2,766)	(434)	2,211
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)	17,503	1,870	(1,736)	2,444
Purchases, issuances and settlements	58,666	96,765	256,847	84,154
Transfers in and/or (out) of Level 3	(3,818)	3,977	(3,818)	1,519
Ending balance	953,190	474,322	953,190	474,322
The amount of total gains or losses for the period included in earnings (or changes in net assets) attributable to the change in unrealized gains or losses relating to assets still held at the reporting date			(539)
Market Value [Member] \| Corporate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	642,432	367,178	546,939	376,250
Total gains or (losses) (realized/unrealized), Included in earnings	(248)	(2,528)	(462)	2,330
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)	(549)	1,870	(3,906)	2,444
Purchases, issuances and settlements	14,346	101,732	113,410	89,686
Transfers in and/or (out) of Level 3	(4,795)	3,977	(4,795)	1,519
Ending balance	651,186	472,229	651,186	472,229
The amount of total gains or losses for the period included in earnings (or changes in net assets) attributable to the change in unrealized gains or losses relating to assets still held at the reporting date			(539)
Market Value [Member] \| Asset-backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	238,631		153,641
Total gains or (losses) (realized/unrealized), Included in earnings	121		125
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)	18,092		2,210
Purchases, issuances and settlements	38,886		139,754
Transfers in and/or (out) of Level 3
Ending balance	295,730		295,730
The amount of total gains or losses for the period included in earnings (or changes in net assets) attributable to the change in unrealized gains or losses relating to assets still held at the reporting date
Market Value [Member] \| Foreign Corporate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		7,298	1,751	7,744
Total gains or (losses) (realized/unrealized), Included in earnings	(97)	(238)	(97)	(119)
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)	(40)		(40)
Purchases, issuances and settlements	5,434	(4,967)	3,683	(5,532)
Transfers in and/or (out) of Level 3	977		977
Ending balance	6,274	2,093	6,274	2,093
The amount of total gains or losses for the period included in earnings (or changes in net assets) attributable to the change in unrealized gains or losses relating to assets still held at the reporting date
Fair Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	4,703	2,350	5,826	2,337
Total gains or (losses) (realized/unrealized), Included in earnings	(272)	356	(1,395)	369
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)
Purchases, issuances and settlements		(2,706)		(2,706)
Transfers in and/or (out) of Level 3
Ending balance	4,431		4,431
The amount of total gains or losses for the period included in earnings (or changes in net assets) attributable to the change in unrealized gains or losses relating to assets still held at the reporting date
Fair Value [Member] \| Foreign Corporate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	4,703	2,350	5,826	2,337
Total gains or (losses) (realized/unrealized), Included in earnings	(272)	356	(1,395)	369
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)
Purchases, issuances and settlements		(2,706)		(2,706)
Transfers in and/or (out) of Level 3
Ending balance	4,431		4,431
The amount of total gains or losses for the period included in earnings (or changes in net assets) attributable to the change in unrealized gains or losses relating to assets still held at the reporting date

Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
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Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

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-Publisher FASB
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Amount of purchases, (sales), issuances and (settlements) of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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