Investments (Amortized cost or cost and fair value of investments) (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost		$ 5,584,599	$ 5,556,900
Short-term investments, at amortized cost		2,796,358	1,941,615
Other investments, at amortized cost		380,130	315,963
Investments in investment affiliates, at cost		84,840	69,794
Total investments, amortized cost		8,845,927	7,884,272
Fixed maturities, at fair value		5,543,030	5,510,331
Short-term investments, at fair value		2,796,170	1,941,635
Other investments, at fair value		405,712	336,856
Investments in investment affiliates		100,431	87,673
Total investments, at fair value		8,845,343	7,876,495
Managed investments
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost		5,427,113	5,369,053
Short-term investments, at amortized cost		228,574	237,349
Other investments, at amortized cost		380,130	315,963
Investments in investment affiliates, at cost	[1]	84,840	69,794
Total investments, amortized cost		6,120,657	5,992,159
Fixed maturities, at fair value		5,384,699	5,323,952
Short-term investments, at fair value		228,386	237,369
Other investments, at fair value		405,712	336,856
Investments in investment affiliates	[1]	100,431	87,673
Total investments, at fair value		6,119,228	5,985,850
Managed investments | U.S. government and government agency
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost		809,392	940,428
Fixed maturities, at fair value		804,126	937,202
Managed investments | Non-U.S. government and government agency
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost		245,651	241,549
Fixed maturities, at fair value		240,791	237,968
Managed investments | U.S. states, municipalities and political subdivisions
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost		271,742	299,929
Fixed maturities, at fair value		271,830	301,289
Managed investments | Agency residential mortgage-backed securities
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost		684,490	606,676
Fixed maturities, at fair value		679,595	610,582
Managed investments | Non-agency residential mortgage-backed securities
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost		15,858	27,025
Fixed maturities, at fair value		15,477	26,920
Managed investments | U.S. corporate
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost		1,540,036	1,503,614
Fixed maturities, at fair value		1,534,508	1,489,951
Managed investments | Non-U.S. corporate
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost		418,520	453,178
Fixed maturities, at fair value		410,227	446,570
Managed investments | Bank loans
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost		579,121	592,981
Fixed maturities, at fair value		570,399	576,211
Managed investments | Asset-backed securities
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost		528,563	440,363
Fixed maturities, at fair value		526,814	437,124
Managed investments | Commercial mortgage backed securities
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost		333,740	263,310
Fixed maturities, at fair value		330,932	260,135
Managed investments | Fund of hedge funds
Amortized cost or cost and fair value of investments
Other investments, at amortized cost		1,457	1,457
Other investments, at fair value		955	1,417
Managed investments | Hedge funds
Amortized cost or cost and fair value of investments
Other investments, at amortized cost		11,292	14,018
Other investments, at fair value		17,381	20,980
Managed investments | Private equity investments
Amortized cost or cost and fair value of investments
Other investments, at amortized cost		72,814	53,489
Other investments, at fair value		89,809	63,771
Managed investments | Fixed income investment funds
Amortized cost or cost and fair value of investments
Other investments, at amortized cost		241,536	188,121
Other investments, at fair value		242,093	188,721
Managed investments | Overseas deposits
Amortized cost or cost and fair value of investments
Other investments, at amortized cost		50,106	54,484
Other investments, at fair value		50,106	54,484
Managed investments | Mutual funds
Amortized cost or cost and fair value of investments
Other investments, at amortized cost		2,925	4,394
Other investments, at fair value		5,368	7,483
Non-managed investments
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost		157,486	187,847
Short-term investments, at amortized cost		2,567,784	1,704,266
Total investments, amortized cost		2,725,270	1,892,113
Fixed maturities, at fair value		158,331	186,379
Short-term investments, at fair value		2,567,784	1,704,266
Total investments, at fair value		2,726,115	1,890,645
Non-managed investments | Catastrophe bonds
Amortized cost or cost and fair value of investments
Fixed maturities, at amortized cost		157,486	187,847
Fixed maturities, at fair value		$ 158,331	$ 186,379

[1] The Company’s investments in investment affiliates have been treated as equity method investments with the corresponding gains and losses recorded inincome as “Income (loss) from investment affiliates.”