Investments (Rollforward of Allowance for Credit Loss) (Details) $ in Thousands	12 Months Ended
Dec. 31, 2020 USD ($)
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Additions for credit losses not previously recorded	$ 94,560
Reduction for securities with credit losses due to intent to sell	(22,436)
Reduction for securities sold during the period	(7,353)
Ending balance	64,771
United States Municipalities, States and Territories
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Additions for credit losses not previously recorded	2,844
Reduction for securities with credit losses due to intent to sell	0
Reduction for securities sold during the period	0
Ending balance	2,844
Corporate Securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Additions for credit losses not previously recorded	60,193
Reduction for securities with credit losses due to intent to sell	0
Reduction for securities sold during the period	0
Ending balance	60,193
Commercial Mortgage Backed Securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Additions for credit losses not previously recorded	29,241
Reduction for securities with credit losses due to intent to sell	(21,888)
Reduction for securities sold during the period	(7,353)
Ending balance	0
Residential Mortgage Backed Securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Additions for credit losses not previously recorded	1,734
Reduction for securities with credit losses due to intent to sell	0
Reduction for securities sold during the period	0
Ending balance	1,734
Other Asset Backed Securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Additions for credit losses not previously recorded	548
Reduction for securities with credit losses due to intent to sell	(548)
Reduction for securities sold during the period	0
Ending balance	$ 0

Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922900-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922900-210455

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aaa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

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