Investments (Components of net investment income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net investment income
Net investment income	$ 48,960	$ 44,458	$ 44,241	$ 40,214	$ 38,153	$ 43,514	$ 39,257	$ 29,461	$ 177,873	$ 150,385	$ 127,824
Managed investments
Net investment income
Total gross investment income									161,624	149,939	128,861
Investment expenses									(8,669)	(8,221)	(7,695)
Net investment income									152,955	141,718	121,166
Managed investments | Fixed maturities and short-term investments
Net investment income
Total gross investment income									127,600	119,085	113,627
Managed investments | Other investments
Net investment income
Total gross investment income									29,930	27,860	13,307
Managed investments | Cash and cash equivalents and restricted cash
Net investment income
Total gross investment income									4,069	2,939	1,911
Managed investments | Securities lending income
Net investment income
Total gross investment income									25	55	16
Non-managed investments
Net investment income
Net investment income									24,918	8,667	6,658
Non-managed investments | Fixed maturities and short-term investments
Net investment income
Total gross investment income									14,833	6,931	6,528
Non-managed investments | Cash and cash equivalents and restricted cash
Net investment income
Total gross investment income									$ 10,085	$ 1,736	$ 130