Investments - Analysis of net investment income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Investment Income [Line Items]
Equity securities and other investments	$ 34,620	$ 28,200	$ 42,252
Interest on funds withheld	10,276	10,835	14,583
Total gross investment income	912,884	952,685	996,374
Investment expenses	(85,751)	(80,315)	(77,749)
Net investment income	827,133	872,370	918,625
Exclude Life Funds Withheld Assets
Net Investment Income [Line Items]
Fixed maturities, short term investments and cash equivalents	713,237	726,161	809,964
Net investment income	672,382	684,881	789,050
Life Funds Withheld Assets
Net Investment Income [Line Items]
Fixed maturities, short term investments and cash equivalents	154,751	187,489	129,575
Net investment income	$ 154,751	$ 187,489	$ 129,575