Investments (Gross Unrealized Losses on Investments, By Category and Length of Time) (Details) - USD ($) $ in Thousands		Sep. 30, 2020		Jun. 30, 2020	[2]	Jan. 01, 2020	[2]	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value		$ 6,103,273						$ 3,259,219
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(414,874)	[1]					(59,699)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		3,226,303						4,448,519
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(390,656)	[1]					(232,356)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		9,329,576						7,707,738
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(805,530)	[1]					(292,055)
Fixed maturity securities, available for sale, allowance for credit losses		61,619		$ 50,186		$ 0
United States Government Full Faith and Credit
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value								144,582
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses								(96)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value								0
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses								0
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value								144,582
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses								(96)
Fixed maturity securities, available for sale, allowance for credit losses		0
United States Government Sponsored Agencies
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value		800,010						168,732
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(672)	[1]					(1,229)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		0						201,444
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		0	[1]					(3,556)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		800,010						370,176
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(672)	[1]					(4,785)
Fixed maturity securities, available for sale, allowance for credit losses		0
United States Municipalities, States and Territories
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value		39,734						285,481
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(5,865)	[1]					(8,173)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		436						3,081
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(1)	[1]					(77)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		40,170						288,562
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(5,866)	[1]					(8,250)
Fixed maturity securities, available for sale, allowance for credit losses		0
Foreign Government Obligations
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value		13,782
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	[1]	(408)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		0
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses	[1]	0
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		13,782
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	[1]	(408)
Fixed maturity securities, available for sale, allowance for credit losses		0
Corporate Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, allowance for credit losses		53,045		46,749		0
Corporate Securities | Finance, Insurance and Real Estate Sectors
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value		325,056						267,521
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(12,818)	[1]					(4,785)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		0						121,993
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		0	[1]					(4,744)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		325,056						389,514
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(12,818)	[1]					(9,529)
Corporate Securities | Manufacturing, Construction and Mining
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value		131,180						161,633
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(8,185)	[1]					(6,039)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		19,391						44,606
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(1,734)	[1]					(3,951)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		150,571						206,239
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(9,919)	[1]					(9,990)
Corporate Securities | Utilities and Related Sectors
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value		409,451						334,635
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(57,131)	[1]					(7,730)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		38,992						51,269
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(6,719)	[1]					(3,482)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		448,443						385,904
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(63,850)	[1]					(11,212)
Corporate Securities | Wholesale/Retail Trade
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value		168,634						54,289
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(16,412)	[1]					(1,751)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		81,245						129,364
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(17,821)	[1]					(9,411)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		249,879						183,653
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(34,233)	[1]					(11,162)
Corporate Securities | Services, Media and Other
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value		404,680						275,135
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(52,252)	[1]					(6,135)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		230,903						316,086
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(61,600)	[1]					(34,231)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		635,583						591,221
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(113,852)	[1]					(40,366)
Residential Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value		208,953						212,404
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(3,200)	[1]					(2,686)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		11,884						11,332
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(1,788)	[1]					(1,005)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		220,837						223,736
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(4,988)	[1]					(3,691)
Fixed maturity securities, available for sale, allowance for credit losses		1,221		777		0
Commercial Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value		1,954,977						602,394
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(193,626)	[1]					(9,366)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		67,940						194,328
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(16,450)	[1]					(4,417)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		2,022,917						796,722
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(210,076)	[1]					(13,783)
Fixed maturity securities, available for sale, allowance for credit losses		7,353		2,660		0
Other Asset Backed Securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, fair value		1,646,816						752,413
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(64,305)	[1]					(11,709)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		2,775,512						3,375,016
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(284,543)	[1]					(167,482)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		4,422,328						4,127,429
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(348,848)	[1]					$ (179,191)
Fixed maturity securities, available for sale, allowance for credit losses		$ 0		$ 0		$ 0

[1] (1) Unrealized losses have not been reduced to reflect the allowance for credit losses of $61.6 million as of September 30, 2020.

[2] (1) The allowance for credit loss associated with available for sale fixed maturity securities was applied prospectively upon adoption of authoritative guidance effective January 1, 2020. See Note 1 for further details.