Investments (Maturity profile of fixed maturity investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Maturity profile
Amortized cost	$ 5,524,027	$ 5,556,900
Fair value	5,551,586	5,510,331
Managed investments
Maturity profile
Amortized cost	5,366,428	5,369,053
Fair value	5,393,525	5,323,952
Managed investments | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	3,941,500	4,031,679
Fair value	3,949,980	3,989,191
Managed investments | Due in one year or less | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	349,115	367,132
Fair value	348,137	366,019
Managed investments | Due after one year through five years | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	2,990,421	2,965,920
Fair value	2,996,295	2,936,053
Managed investments | Due after five years through ten years | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	485,899	548,183
Fair value	492,101	539,083
Managed investments | Due after ten years | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	116,065	150,444
Fair value	113,447	148,036
Managed investments | Asset backed and mortgaged backed securities | No single maturity date
Maturity profile
Amortized cost	1,424,928	1,337,374
Fair value	1,443,545	1,334,761
Non-managed investments
Maturity profile
Amortized cost	157,599	187,847
Fair value	158,061	186,379
Non-managed investments | Due in one year or less | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	28,445	7,504
Fair value	29,328	7,544
Non-managed investments | Due after one year through five years | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	128,614	165,093
Fair value	128,185	163,575
Non-managed investments | Due after five years through ten years | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	540	15,250
Fair value	548	15,260
Non-managed investments | Due after ten years | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	0	0
Fair value	$ 0	$ 0