Investments (Gross unrealized gains and losses on fixed maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost or cost	$ 5,547,838	$ 5,556,900
Short-term investments, at amortized cost or cost	2,481,573	1,941,615
Other investments, at amortized cost or cost	371,668	315,963
Total investments	8,401,079	7,814,478
Gross unrealized gains	85,914	36,781
Gross unrealized losses	(34,551)	(62,437)
Total fixed maturities	5,576,341	5,510,331
Total short-term investments	2,481,406	1,941,635
Total other investments	394,695	336,856
Total investments	8,452,442	7,788,822
Managed investments
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost or cost	5,392,839	5,369,053
Short-term investments, at amortized cost or cost	197,970	237,349
Other investments, at amortized cost or cost	371,668	315,963
Total investments	5,962,477	5,922,365
Fixed maturities, gross unrealized gains	57,659	12,724
Short term investments, gross unrealized gains	0	20
Other investments, gross unrealized gains	25,365	23,402
Gross unrealized gains	83,024	36,146
Fixed maturities, gross unrealized losses	(30,548)	(57,825)
Short term investments, gross unrealized losses	(167)	0
Other investments, gross unrealized losses	(2,338)	(2,509)
Gross unrealized losses	(33,053)	(60,334)
Total fixed maturities	5,419,950	5,323,952
Total short-term investments	197,803	237,369
Total other investments	394,695	336,856
Total investments	6,012,448	5,898,177
Managed investments | U.S. government and government agency
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost or cost	833,897	940,428
Fixed maturities, gross unrealized gains	4,088	333
Fixed maturities, gross unrealized losses	(439)	(3,559)
Total fixed maturities	837,546	937,202
Managed investments | Non-U.S. government and government agency
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost or cost	251,863	241,549
Fixed maturities, gross unrealized gains	1,503	257
Fixed maturities, gross unrealized losses	(4,976)	(3,838)
Total fixed maturities	248,390	237,968
Managed investments | U.S. states, municipalities and political subdivisions
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost or cost	278,172	299,929
Fixed maturities, gross unrealized gains	5,585	2,322
Fixed maturities, gross unrealized losses	(396)	(962)
Total fixed maturities	283,361	301,289
Managed investments | Agency residential mortgage-backed securities
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost or cost	644,403	606,676
Fixed maturities, gross unrealized gains	13,980	6,361
Fixed maturities, gross unrealized losses	(507)	(2,455)
Total fixed maturities	657,876	610,582
Managed investments | Non-agency residential mortgage-backed securities
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost or cost	21,142	27,025
Fixed maturities, gross unrealized gains	160	310
Fixed maturities, gross unrealized losses	(687)	(415)
Total fixed maturities	20,615	26,920
Managed investments | U.S. corporate
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost or cost	1,509,117	1,503,614
Fixed maturities, gross unrealized gains	19,102	1,594
Fixed maturities, gross unrealized losses	(1,542)	(15,257)
Total fixed maturities	1,526,677	1,489,951
Managed investments | Non-U.S. corporate
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost or cost	434,621	453,178
Fixed maturities, gross unrealized gains	3,611	797
Fixed maturities, gross unrealized losses	(6,600)	(7,405)
Total fixed maturities	431,632	446,570
Managed investments | Bank loans
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost or cost	598,847	592,981
Fixed maturities, gross unrealized gains	1,635	275
Fixed maturities, gross unrealized losses	(12,945)	(17,045)
Total fixed maturities	587,537	576,211
Managed investments | Asset-backed securities
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost or cost	484,480	440,363
Fixed maturities, gross unrealized gains	2,607	344
Fixed maturities, gross unrealized losses	(1,568)	(3,583)
Total fixed maturities	485,519	437,124
Managed investments | Commercial mortgage-backed securities
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost or cost	336,297	263,310
Fixed maturities, gross unrealized gains	5,388	131
Fixed maturities, gross unrealized losses	(888)	(3,306)
Total fixed maturities	340,797	260,135
Managed investments | Fund of hedge funds
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Other investments, at amortized cost or cost	1,457	1,457
Other investments, gross unrealized gains	0	0
Other investments, gross unrealized losses	(498)	(40)
Total other investments	959	1,417
Managed investments | Hedge funds
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Other investments, at amortized cost or cost	12,073	14,018
Other investments, gross unrealized gains	5,983	6,962
Other investments, gross unrealized losses	0	0
Total other investments	18,056	20,980
Managed investments | Private equity investments
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Other investments, at amortized cost or cost	69,353	53,489
Other investments, gross unrealized gains	16,157	12,751
Other investments, gross unrealized losses	(1,840)	(2,469)
Total other investments	83,670	63,771
Managed investments | Fixed income investment funds
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Other investments, at amortized cost or cost	232,614	188,121
Other investments, gross unrealized gains	823	600
Other investments, gross unrealized losses	0	0
Total other investments	233,437	188,721
Managed investments | Overseas deposits
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Other investments, at amortized cost or cost	53,246	54,484
Other investments, gross unrealized gains	0	0
Other investments, gross unrealized losses	0	0
Total other investments	53,246	54,484
Managed investments | Mutual funds
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Other investments, at amortized cost or cost	2,925	4,394
Other investments, gross unrealized gains	2,402	3,089
Other investments, gross unrealized losses	0	0
Total other investments	5,327	7,483
Non-managed investments
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost or cost	154,999	187,847
Short-term investments, at amortized cost or cost	2,283,603	1,704,266
Total investments	2,438,602	1,892,113
Short term investments, gross unrealized gains	0	0
Gross unrealized gains	2,890	635
Short term investments, gross unrealized losses	0	0
Gross unrealized losses	(1,498)	(2,103)
Total fixed maturities	156,391	186,379
Total short-term investments	2,283,603	1,704,266
Total investments	2,439,994	1,890,645
Non-managed investments | Catastrophe bonds
Amortized cost (or cost), gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost or cost	154,999	187,847
Fixed maturities, gross unrealized gains	2,890	635
Fixed maturities, gross unrealized losses	(1,498)	(2,103)
Total fixed maturities	$ 156,391	$ 186,379