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Investment Information - Aging of available for sale securities in an unrealized loss position (Details) - USD ($) $ in Thousands		Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months		$ 4,671,564	$ 4,280,273
Estimated Fair Value - 12 Months or More		43,555	158,538
Estimated Fair Value		4,715,119	4,438,811
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months		(130,799)	(52,926)
Gross Unrealized Losses - 12 Months or More		(4,384)	(2,077)
Gross Unrealized Losses		(135,183)	(55,003)
Fixed maturities
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	[1]	3,645,571	4,184,496
Estimated Fair Value - 12 Months or More	[1]	43,555	158,538
Estimated Fair Value	[1]	3,689,126	4,343,034
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(130,165)	(51,378)
Gross Unrealized Losses - 12 Months or More	[1]	(4,384)	(2,077)
Gross Unrealized Losses	[1]	(134,549)	(53,455)
Fixed maturities | Corporate bonds
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	[1]	737,840	675,131
Estimated Fair Value - 12 Months or More	[1]	8,726	37,671
Estimated Fair Value	[1]	746,566	712,802
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(32,848)	(12,350)
Gross Unrealized Losses - 12 Months or More	[1]	(1,852)	(443)
Gross Unrealized Losses	[1]	(34,700)	(12,793)
Fixed maturities | Mortgage backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	[1]	150,547	102,887
Estimated Fair Value - 12 Months or More	[1]	108	203
Estimated Fair Value	[1]	150,655	103,090
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(5,212)	(927)
Gross Unrealized Losses - 12 Months or More	[1]	(33)	(4)
Gross Unrealized Losses	[1]	(5,245)	(931)
Fixed maturities | Municipal bonds
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	[1]	46,969	220,296
Estimated Fair Value - 12 Months or More	[1]	0	0
Estimated Fair Value	[1]	46,969	220,296
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(542)	(2,213)
Gross Unrealized Losses - 12 Months or More	[1]	0	0
Gross Unrealized Losses	[1]	(542)	(2,213)
Fixed maturities | Commercial mortgage backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	[1]	205,336	147,290
Estimated Fair Value - 12 Months or More	[1]	2,523	2,683
Estimated Fair Value	[1]	207,859	149,973
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(7,858)	(2,302)
Gross Unrealized Losses - 12 Months or More	[1]	(147)	(28)
Gross Unrealized Losses	[1]	(8,005)	(2,330)
Fixed maturities | US government and government agencies
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	[1]	412,319	1,373,127
Estimated Fair Value - 12 Months or More	[1]	0	32,058
Estimated Fair Value	[1]	412,319	1,405,185
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(2,708)	(10,089)
Gross Unrealized Losses - 12 Months or More	[1]	0	(45)
Gross Unrealized Losses	[1]	(2,708)	(10,134)
Fixed maturities | Non-US government securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	[1]	1,371,109	1,224,243
Estimated Fair Value - 12 Months or More	[1]	0	37,610
Estimated Fair Value	[1]	1,371,109	1,261,853
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(43,928)	(20,163)
Gross Unrealized Losses - 12 Months or More	[1]	0	(167)
Gross Unrealized Losses	[1]	(43,928)	(20,330)
Fixed maturities | Asset backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	[1]	721,451	441,522
Estimated Fair Value - 12 Months or More	[1]	32,198	48,313
Estimated Fair Value	[1]	753,649	489,835
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(37,069)	(3,334)
Gross Unrealized Losses - 12 Months or More	[1]	(2,352)	(1,390)
Gross Unrealized Losses	[1]	(39,421)	(4,724)
Short-term investments
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months		1,025,993	95,777
Estimated Fair Value - 12 Months or More		0	0
Estimated Fair Value		1,025,993	95,777
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months		(634)	(1,548)
Gross Unrealized Losses - 12 Months or More		0	0
Gross Unrealized Losses		$ (634)	$ (1,548)
[1] In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.”

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