Investment Information - Net investment income (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Net investment income:
Gross investment income		$ 134,016	$ 109,341	$ 272,391	$ 220,700
Investment expenses		(22,892)	(21,003)	(43,393)	(38,627)
Net investment income		111,124	88,338	228,998	182,073
Fixed maturities
Net investment income:
Gross investment income		94,270	77,994	188,663	151,667
Term loan investments
Net investment income:
Gross investment income		19,105	18,608	40,275	38,620
Equity securities (dividends)
Net investment income:
Gross investment income		3,654	3,663	6,297	7,098
Short-term investments
Net investment income:
Gross investment income		2,016	816	3,775	1,312
Other
Net investment income:
Gross investment income	[1]	$ 14,971	$ 8,260	$ 33,381	$ 22,003

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