Fixed Maturity Securities In An Unrealized Loss Position (Non-Investment Grade Fixed Maturity Securities) (Details) $ in Thousands	Sep. 30, 2021 USD ($) position	Dec. 31, 2020 USD ($)
Corporate
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	$ 1,519,705	$ 652,162
Gross Unrealized Loss	13,662	8,480
State and municipal
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	382,192	168,282
Gross Unrealized Loss	6,335	2,069
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	403,780	236,331
Gross Unrealized Loss	7,295	5,351
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	3,478,923	2,046,010
Gross Unrealized Loss	$ 14,201	$ 33,497
Noninvestment Grade Investments at Loss Position
Debt Securities, Available-for-sale [Line Items]
Number of Securities \| position	51
Aggregate Fair Value	$ 208,800
Gross Unrealized Loss	$ 34,281
Noninvestment Grade Investments at Loss Position \| Foreign government
Debt Securities, Available-for-sale [Line Items]
Number of Securities \| position	31
Aggregate Fair Value	$ 119,141
Gross Unrealized Loss	$ 32,048
Noninvestment Grade Investments at Loss Position \| Corporate
Debt Securities, Available-for-sale [Line Items]
Number of Securities \| position	13
Aggregate Fair Value	$ 49,623
Gross Unrealized Loss	$ 1,879
Noninvestment Grade Investments at Loss Position \| State and municipal
Debt Securities, Available-for-sale [Line Items]
Number of Securities \| position	2
Aggregate Fair Value	$ 39,693
Gross Unrealized Loss	$ 335
Noninvestment Grade Investments at Loss Position \| Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Number of Securities \| position	4
Aggregate Fair Value	$ 249
Gross Unrealized Loss	$ 16
Noninvestment Grade Investments at Loss Position \| Asset-backed
Debt Securities, Available-for-sale [Line Items]
Number of Securities \| position	1
Aggregate Fair Value	$ 94
Gross Unrealized Loss	$ 3

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922888-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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