Fair Value (Activity Under Level 3, Fair Value Measurements Using Significant Unobservable Inputs By Asset Type) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Market Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 702,331	$ 383,994
Total gains or (losses) (realized/unrealized), Included in earnings	1,772	4,925
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)	10,710	3,501
Purchases, issuances and settlements	545,687	305,216
Transfers in and/or (out) of Level 3	(924)	4,695
Ending balance	1,259,576	702,331
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	546,939	376,250
Total gains or (losses) (realized/unrealized), Included in earnings	1,216	4,937
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)	(1,115)	(21)
Purchases, issuances and settlements	84,840	161,078
Transfers in and/or (out) of Level 3	(1,037)	4,695
Ending balance	630,843	546,939
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	153,641
Total gains or (losses) (realized/unrealized), Included in earnings	681
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)	11,678	3,632
Purchases, issuances and settlements	457,033	150,009
Transfers in and/or (out) of Level 3
Ending balance	623,033	153,641
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	1,751	7,744
Total gains or (losses) (realized/unrealized), Included in earnings	(125)	(12)
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)	147	(110)
Purchases, issuances and settlements	3,814	(5,871)
Transfers in and/or (out) of Level 3	113
Ending balance	5,700	1,751
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	2,337
Total gains or (losses) (realized/unrealized), Included in earnings	(919)	2,163
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)
Purchases, issuances and settlements	(4,907)	1,326
Transfers in and/or (out) of Level 3
Ending balance		5,826
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		1,796
Fair Value [Member] \| Foreign Corporate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	5,826	2,337
Total gains or (losses) (realized/unrealized), Included in earnings	(919)	2,163
Total gains or (losses) (realized/unrealized), Included in other comprehensive income (loss)
Purchases, issuances and settlements	(4,907)	1,326
Transfers in and/or (out) of Level 3
Ending balance		5,826
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		$ 1,796

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
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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
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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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