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Investments - Net Realized and Unrealized Gains (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net realized gains (losses) on sale:
Total net realized gains (losses) on sale									$ 96,328	$ (27,633)	$ 1,668
Net unrealized gains (losses):
Fixed maturity securities, trading									349,975	(165,187)
Total net unrealized gains (losses)									935,023	(385,251)	188,666
Net realized gains (losses) and change in fair value of funds held- directly managed	$ 152,664	$ 148,185	$ 269,711	$ 460,791	$ (158,213)	$ (57,223)	$ (54,418)	$ (143,030)	1,031,351	(412,884)	190,334
Proceeds from sale of available from sale securities									310,400	11,400	40,800
Fixed maturity investments
Net realized gains (losses) on sale:
Gross realized gains on fixed maturity securities, available-for-sale securities									4,856	27	616
Gross realized losses on fixed maturity securities, available-for-sale securities									(962)	(90)	(125)
Net realized gains (losses) on trading securities									91,313	(27,646)	4,695
Net unrealized gains (losses):
Fixed maturity securities, trading											35,878
Funds Held - Directly Managed
Net realized gains (losses) on sale:
Net realized gains (losses) on trading securities									1,495	(3,940)	(4,219)
Net unrealized gains (losses):
Fixed maturity securities, trading									88,053	(46,257)
Fixed maturity securities, trading											33,902
Equity investments
Net realized gains (losses) on sale:
Net realized gains (losses) on equity investments, trading									(374)	4,016	701
Net unrealized gains (losses):
Equity investments, trading									55,359	(9,831)	16,498
Other investments
Net unrealized gains (losses):
Other investments									$ 441,636	$ (163,976)	$ 102,388