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Investment Information - Net Investment Income (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net investment income:
Gross investment income										$ 608,823	$ 716,643	$ 655,727
Investment expenses										(89,215)	(88,905)	(92,094)
Net investment income		$ 114,458	$ 128,512	$ 131,485	$ 145,153	$ 154,263	$ 161,488	$ 155,038	$ 156,949	519,608	627,738	563,633
Fixed maturities
Net investment income:
Gross investment income										412,481	505,399	470,912
Term loan investments
Net investment income:
Gross investment income										84,149	98,949	87,926
Equity securities
Net investment income:
Gross investment income										28,958	15,857	13,154
Short-term investments
Net investment income:
Gross investment income										10,840	15,820	18,793
Other
Net investment income:
Gross investment income	[1]									$ 72,395	$ 80,618	$ 64,942
[1] Includes income distributions from investment funds and other items.

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Amount before accretion (amortization) of purchase discount (premium) of interest income and dividend income on nonoperating securities.

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