Investments (Amortized cost or cost and fair value of investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost	$ 5,876,261	$ 5,584,599
Short-term investments, at amortized cost	3,381,714	2,796,358
Other investments, at amortized cost	330,416	380,130
Investments in investment affiliates, at cost	61,944	84,840
Total investments, amortized cost	9,650,335	8,845,927
Fixed maturities, at fair value	5,858,348	5,543,030
Short-term investments trading, at fair value	3,381,757	2,796,170
Other investments, at fair value	355,218	405,712
Investments in investment affiliates	100,137	100,431
Total investments, at fair value	9,695,460	8,845,343
Managed investments
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost	5,644,929	5,427,113
Short-term investments, at amortized cost	229,968	228,574
Other investments, at amortized cost	330,416	380,130
Total investments, amortized cost	6,267,257	6,120,657
Fixed maturities, at fair value	5,628,654	5,384,699
Short-term investments trading, at fair value	230,011	228,386
Other investments, at fair value	355,218	405,712
Total investments, at fair value	6,314,020	6,119,228
Managed investments | U.S. government and government agency
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost	733,510	809,392
Fixed maturities, at fair value	727,397	804,126
Managed investments | Non-U.S. government and government agency
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost	310,845	245,651
Fixed maturities, at fair value	312,239	240,791
Managed investments | U.S. states, municipalities and political subdivisions
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost	201,347	271,742
Fixed maturities, at fair value	201,303	271,830
Managed investments | Agency residential mortgage-backed securities
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost	984,387	684,490
Fixed maturities, at fair value	978,049	679,595
Managed investments | Non-agency residential mortgage-backed securities
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost	40,264	15,858
Fixed maturities, at fair value	40,373	15,477
Managed investments | U.S. corporate
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost	1,531,498	1,540,036
Fixed maturities, at fair value	1,533,395	1,534,508
Managed investments | Non-U.S. corporate
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost	420,522	418,520
Fixed maturities, at fair value	422,249	410,227
Managed investments | Bank loans
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost	450,320	579,121
Fixed maturities, at fair value	442,951	570,399
Managed investments | Asset-backed securities
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost	657,234	528,563
Fixed maturities, at fair value	658,303	526,814
Managed investments | Commercial mortgage backed securities
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost	315,002	333,740
Fixed maturities, at fair value	312,395	330,932
Managed investments | Fund of hedge funds
Amortized cost or cost and fair value of investments
Other investments, at amortized cost	0	1,457
Other investments, at fair value	0	955
Managed investments | Hedge funds
Amortized cost or cost and fair value of investments
Other investments, at amortized cost	6,954	11,292
Other investments, at fair value	15,774	17,381
Managed investments | Private equity investments
Amortized cost or cost and fair value of investments
Other investments, at amortized cost	63,684	66,383
Other investments, at fair value	78,407	82,627
Managed investments | Fixed income investment funds
Amortized cost or cost and fair value of investments
Other investments, at amortized cost	203,167	247,967
Other investments, at fair value	204,426	249,275
Managed investments | Overseas deposits
Amortized cost or cost and fair value of investments
Other investments, at amortized cost	56,611	50,106
Other investments, at fair value	56,611	50,106
Managed investments | Mutual funds
Amortized cost or cost and fair value of investments
Other investments, at amortized cost	0	2,925
Other investments, at fair value	0	5,368
Non-managed investments
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost	231,332	157,486
Short-term investments, at amortized cost	3,151,746	2,567,784
Total investments, amortized cost	3,383,078	2,725,270
Fixed maturities, at fair value	229,694	158,331
Short-term investments trading, at fair value	3,151,746	2,567,784
Total investments, at fair value	3,381,440	2,726,115
Non-managed investments | Catastrophe bonds
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost	231,332	157,486
Fixed maturities, at fair value	$ 229,694	$ 158,331