Investments (Components of net investment income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net investment income
Net investment income	$ 38,153	$ 43,514	$ 39,257	$ 29,461	$ 31,612	$ 31,572	$ 33,611	$ 31,029	$ 150,385	$ 127,824	$ 100,086
Managed investments
Net investment income
Total gross investment income									149,939	128,861	103,272
Investment expenses									(8,221)	(7,695)	(7,472)
Net investment income									141,718	121,166	95,800
Managed investments | Fixed maturities and short-term investments
Net investment income
Total gross investment income									119,085	113,627	93,044
Managed investments | Other investments
Net investment income
Total gross investment income									27,860	13,307	5,111
Managed investments | Cash and cash equivalents and restricted cash
Net investment income
Total gross investment income									2,939	1,911	5,106
Managed investments | Securities lending income
Net investment income
Total gross investment income									55	16	11
Non-managed investments
Net investment income
Net investment income									8,667	6,658	4,286
Non-managed investments | Fixed maturities and short-term investments
Net investment income
Total gross investment income									6,931	6,528	4,204
Non-managed investments | Cash and cash equivalents and restricted cash
Net investment income
Total gross investment income									$ 1,736	$ 130	$ 82