Investments - Net Investment Income (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 Investment Income [Line Items]
Gross investment income									$ 336,534	$ 280,392	$ 219,823
Investment expenses									(15,258)	(9,721)	(11,034)
Net investment income	$ 79,376	$ 85,472	$ 77,732	$ 78,696	$ 68,453	$ 69,430	$ 66,469	$ 66,319	321,276	270,671	208,789
Fixed maturity investments, trading
Net Investment Income [Line Items]
Gross investment income									233,310	189,000	144,367
Short-term investments and cash and cash equivalents
Net Investment Income [Line Items]
Gross investment income									16,607	12,117	9,314
Funds held
Net Investment Income [Line Items]
Gross investment income									19,981	10,041	601
Funds held – directly managed
Net Investment Income [Line Items]
Gross investment income									38,173	37,623	32,479
Investment income from fixed maturities and cash and cash equivalents
Net Investment Income [Line Items]
Gross investment income									308,071	248,781	186,761
Equity investments
Net Investment Income [Line Items]
Gross investment income									16,671	5,397	4,355
Other investments
Net Investment Income [Line Items]
Gross investment income									11,792	19,703	14,337
Life settlements and other
Net Investment Income [Line Items]
Gross investment income									0	6,511	14,370
Investment income from equities and other investments
Net Investment Income [Line Items]
Gross investment income									$ 28,463	$ 31,611	$ 33,062