Fair Value (Activity Under Level 3, Fair Value Measurements Using Significant Unobservable Inputs By Asset Type) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Market Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 1,259,576	$ 702,331
Total gains or (losses) (realized/unrealized), Included in earnings	(5,954)	(210)
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)	(251)	(19,239)
Purchases, issuances and settlements	171,480	198,181
Transfers in and/or (out) of Level 3
Ending balance	1,424,851	881,063
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	630,843	546,939
Total gains or (losses) (realized/unrealized), Included in earnings	(1,788)	(214)
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)	2,835	(3,357)
Purchases, issuances and settlements	2,003	99,064
Transfers in and/or (out) of Level 3
Ending balance	633,893	642,432
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	623,033	153,641
Total gains or (losses) (realized/unrealized), Included in earnings	(4,168)	4
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)	(3,135)	(15,882)
Purchases, issuances and settlements	169,630	100,868
Transfers in and/or (out) of Level 3
Ending balance	785,360	238,631
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	5,700	1,751
Total gains or (losses) (realized/unrealized), Included in earnings	2
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)	49
Purchases, issuances and settlements	(153)	(1,751)
Transfers in and/or (out) of Level 3
Ending balance	5,598
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		5,826
Total gains or (losses) (realized/unrealized), Included in earnings		(1,123)
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)
Purchases, issuances and settlements
Transfers in and/or (out) of Level 3
Ending balance		4,703
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		5,826
Total gains or (losses) (realized/unrealized), Included in earnings		(1,123)
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)
Purchases, issuances and settlements
Transfers in and/or (out) of Level 3
Ending balance		4,703
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)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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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
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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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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-Publisher FASB
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