Investments (Gross Unrealized Losses on Investments, By Category and Length of Time) (Details) - USD ($) $ in Thousands		Jun. 30, 2020		Mar. 31, 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		$ 7,289,479						$ 3,259,219
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(567,058)	[1]					(59,699)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		3,174,156						4,448,519
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(470,819)	[1]					(232,356)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		10,463,635						7,707,738
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(1,037,877)	[1]					(292,055)
Fixed maturity securities, available for sale, allowance for credit losses		50,186		$ 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		51,931						285,481
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(2,039)	[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		51,931						288,562
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(2,039)	[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		23,057
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses	[1]	(1,128)
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		23,057
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses	[1]	(1,128)
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		46,749		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		626,564						267,521
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(23,749)	[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		626,564						389,514
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(23,749)	[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		280,272						161,633
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(12,977)	[1]					(6,039)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		18,451						44,606
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(2,678)	[1]					(3,951)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		298,723						206,239
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(15,655)	[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		457,237						334,635
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(62,364)	[1]					(7,730)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		37,346						51,269
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(9,405)	[1]					(3,482)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		494,583						385,904
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(71,769)	[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		185,882						54,289
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(21,316)	[1]					(1,751)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		105,576						129,364
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(27,228)	[1]					(9,411)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		291,458						183,653
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(48,544)	[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		570,547						275,135
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(64,840)	[1]					(6,135)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		234,595						316,086
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(69,039)	[1]					(34,231)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		805,142						591,221
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(133,879)	[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		221,327						212,404
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(3,882)	[1]					(2,686)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		12,675						11,332
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(1,732)	[1]					(1,005)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		234,002						223,736
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(5,614)	[1]					(3,691)
Fixed maturity securities, available for sale, allowance for credit losses		777		0		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		2,711,400						602,394
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(249,494)	[1]					(9,366)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		88,303						194,328
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(20,115)	[1]					(4,417)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		2,799,703						796,722
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(269,609)	[1]					(13,783)
Fixed maturity securities, available for sale, allowance for credit losses		2,660		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		2,161,262						752,413
Fixed maturity securities, available for sale, continuous unrealized loss position, less than 12 months, unrealized losses		(125,269)	[1]					(11,709)
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, fair value		2,677,210						3,375,016
Fixed maturity securities, available for sale, continuous unrealized loss position, 12 months or more, unrealized losses		(340,622)	[1]					(167,482)
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, fair value		4,838,472						4,127,429
Fixed maturity securities, available for sale securities, continuous unrealized loss position, total, unrealized losses		(465,891)	[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 $50.2 million as of June 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.