Investments (Gross Unrealized Losses on Investments, By Category and Length of Time) (Details) - USD ($) $ in Thousands		Mar. 31, 2020		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		$ 14,807,510				$ 3,259,219
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(1,392,795)	[1]			(59,699)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		2,641,615				4,448,519
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(1,000,487)	[1]			(232,356)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		17,449,125				7,707,738
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(2,393,282)	[1]			(292,055)
Fixed maturity securities, available for sale, allowance for credit losses		28,332		$ 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						168,732
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses						(1,229)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value						201,444
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses						(3,556)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value						370,176
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses						(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		209,291				285,481
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(4,130)	[1]			(8,173)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		0				3,081
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		0	[1]			(77)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		209,291				288,562
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(4,130)	[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		60,079
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	[1]	(4,465)
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		60,079
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	[1]	(4,465)
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		28,332		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		1,970,305				267,521
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(109,704)	[1]			(4,785)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		3,547				121,993
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(372)	[1]			(4,744)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		1,973,852				389,514
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(110,076)	[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		1,154,320				161,633
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(60,623)	[1]			(6,039)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		24,866				44,606
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(6,705)	[1]			(3,951)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		1,179,186				206,239
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(67,328)	[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		1,375,023				334,635
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(87,628)	[1]			(7,730)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		43,307				51,269
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(10,413)	[1]			(3,482)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		1,418,330				385,904
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(98,041)	[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		643,529				54,289
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(63,887)	[1]			(1,751)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		88,471				129,364
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(44,320)	[1]			(9,411)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		732,000				183,653
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(108,207)	[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		2,683,039				275,135
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(366,658)	[1]			(6,135)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		183,159				316,086
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(105,860)	[1]			(34,231)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		2,866,198				591,221
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(472,518)	[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		740,561				212,404
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(23,072)	[1]			(2,686)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		10,375				11,332
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(1,533)	[1]			(1,005)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		750,936				223,736
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(24,605)	[1]			(3,691)
Fixed maturity securities, available for sale, allowance for credit losses		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		3,979,173				602,394
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(296,598)	[1]			(9,366)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		94,240				194,328
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(14,299)	[1]			(4,417)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		4,073,413				796,722
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(310,897)	[1]			(13,783)
Fixed maturity securities, available for sale, allowance for credit losses		0		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		1,992,190				752,413
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(376,030)	[1]			(11,709)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		2,193,650				3,375,016
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(816,985)	[1]			(167,482)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		4,185,840				4,127,429
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(1,193,015)	[1]			$ (179,191)
Fixed maturity securities, available for sale, allowance for credit losses		$ 0		$ 0

[1] (1) Unrealized losses have not been reduced to reflect the allowance for credit losses of $28.3 million as of March 31, 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.