Investments - Unrealized Gain (Loss) on Investment (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Gain (Loss) on Securities [Line Items]
Change in investment gains (losses) on	$ 1,510,033	$ 1,958,165	$ (1,434,918)
Fixed maturities available for sale
Gain (Loss) on Securities [Line Items]
Change in investment gains (losses) on	1,528,339	1,946,910	(1,429,763)
Other investments
Gain (Loss) on Securities [Line Items]
Change in investment gains (losses) on	$ (18,306)	$ 11,255	$ (5,155)

Amount, before tax and after adjustment, of unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) and unrealized gain (loss) on investment in debt security measured at amortized cost (held-to-maturity) from transfer to available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669646-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669646-108580

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=121641772&loc=d3e637-108580

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration