Investments (Maturity profile of fixed maturity investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Maturity profile
Amortized cost	$ 5,584,599	$ 5,556,900
Estimated fair value	5,543,030	5,510,331
Managed investments
Maturity profile
Amortized cost	5,427,113	5,369,053
Estimated fair value	5,384,699	5,323,952
Managed investments | Due in one year or less [Member]
Maturity profile
Amortized cost	350,733	367,132
Estimated fair value	346,161	366,019
Managed investments | Due after one year through five years [Member]
Maturity profile
Amortized cost	2,954,856	2,965,920
Estimated fair value	2,933,146	2,936,053
Managed investments | Due after five years through ten years [Member]
Maturity profile
Amortized cost	430,365	548,183
Estimated fair value	426,647	539,083
Managed investments | Due after ten years [Member]
Maturity profile
Amortized cost	128,508	150,444
Estimated fair value	125,927	148,036
Managed investments | Fixed Maturities Trading Securities Allocated To Contractual Maturity [Member]
Maturity profile
Amortized cost	3,864,462	4,031,679
Estimated fair value	3,831,881	3,989,191
Managed investments | Asset backed and mortgaged backed securities [Member]
Maturity profile
Amortized cost	1,562,651	1,337,374
Estimated fair value	1,552,818	1,334,761
Non-managed investments
Maturity profile
Amortized cost	157,486	187,847
Estimated fair value	158,331	186,379
Non-managed investments | Due in one year or less [Member]
Maturity profile
Amortized cost	43,664	7,504
Estimated fair value	45,418	7,544
Non-managed investments | Due after one year through five years [Member]
Maturity profile
Amortized cost	112,572	165,093
Estimated fair value	111,656	163,575
Non-managed investments | Due after five years through ten years [Member]
Maturity profile
Amortized cost	1,250	15,250
Estimated fair value	1,257	15,260
Non-managed investments | Due after ten years [Member]
Maturity profile
Amortized cost	0	0
Estimated fair value	$ 0	$ 0