Investments (Maturity profile of fixed maturity investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Maturity profile
Amortized cost	$ 5,547,838	$ 5,556,900
Fair value	5,576,341	5,510,331
Managed investments
Maturity profile
Amortized cost	5,392,839	5,369,053
Fair value	5,419,950	5,323,952
Managed investments | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	3,906,517	4,031,679
Fair value	3,915,143	3,989,191
Managed investments | Due in one year or less | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	368,627	367,132
Fair value	366,437	366,019
Managed investments | Due after one year through five years | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	2,990,875	2,965,920
Fair value	2,992,940	2,936,053
Managed investments | Due after five years through ten years | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	433,478	548,183
Fair value	441,449	539,083
Managed investments | Due after ten years | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	113,537	150,444
Fair value	114,317	148,036
Managed investments | Asset backed and mortgaged backed securities | No single maturity date
Maturity profile
Amortized cost	1,486,322	1,337,374
Fair value	1,504,807	1,334,761
Non-managed investments
Maturity profile
Amortized cost	154,999	187,847
Fair value	156,391	186,379
Non-managed investments | Due in one year or less | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	40,231	7,504
Fair value	41,648	7,544
Non-managed investments | Due after one year through five years | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	114,018	165,093
Fair value	113,992	163,575
Non-managed investments | Due after five years through ten years | Fixed maturities trading securities allocated to contractual maturity
Maturity profile
Amortized cost	750	15,250
Fair value	751	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