Investment Information - Net investment income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net investment income:
Gross investment income		$ 150,854	$ 181,655	$ 471,145	$ 540,931
Investment expenses		(22,342)	(20,167)	(65,995)	(67,456)
Net investment income		128,512	161,488	405,150	473,475
Fixed maturities
Net investment income:
Gross investment income		98,344	126,889	318,582	381,706
Term loans
Net investment income:
Gross investment income		22,459	24,236	66,141	73,582
Equity securities
Net investment income:
Gross investment income		6,659	3,992	18,885	11,348
Short-term investments
Net investment income:
Gross investment income		1,332	3,834	9,611	11,872
Other
Net investment income:
Gross investment income	[1]	$ 22,060	$ 22,704	$ 57,926	$ 62,423

[1] Includes income distributions from investment funds and other items.