Investments (Details 4) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	$ 5,280,246	$ 5,063,873
Gross unrealized gains	83,612	90,039
Gross unrealized losses	(16,320)	(35,053)
Estimated fair value	5,347,538	5,118,859
U.S. Government and Government Agency [Member]
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	828,627	1,665,050
Gross unrealized gains	10,656	20,134
Gross unrealized losses	(371)	(8,018)
Estimated fair value	838,912	1,677,166
Non-US Government and Government Agency [Member]
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	470,438	550,759
Gross unrealized gains	9,551	11,635
Gross unrealized losses	(3,399)	(8,195)
Estimated fair value	476,590	554,199
States, municipalities, political subdivision [Member]
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	29,199	26,365
Gross unrealized gains	394	90
Gross unrealized losses	(17)	(170)
Estimated fair value	29,576	26,285
Agency residential mortgage-backed securities [Member]
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	454,517	430,873
Gross unrealized gains	16,899	15,491
Gross unrealized losses	(483)	(505)
Estimated fair value	470,933	445,859
Non-Agency residential mortgage-backed securities [Member]
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	57,678	62,020
Gross unrealized gains	148	64
Gross unrealized losses	(6,603)	(5,614)
Estimated fair value	51,223	56,470
U.S. corporate [Member]
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	1,378,760	1,288,078
Gross unrealized gains	29,227	28,526
Gross unrealized losses	(1,396)	(8,198)
Estimated fair value	1,406,591	1,308,406
Non-U.S. corporate [Member]
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	618,411	497,689
Gross unrealized gains	10,615	7,939
Gross unrealized losses	(981)	(3,561)
Estimated fair value	628,045	502,067
Bank loans [Member]
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	389,193	52,612
Gross unrealized gains	702	58
Gross unrealized losses	(2,694)	(104)
Estimated fair value	387,201	52,566
Catastrophe bonds [Member]
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	29,550	56,991
Gross unrealized gains	445	2,042
Gross unrealized losses	(61)	(296)
Estimated fair value	29,934	58,737
Asset-backed securities [Member]
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	275,417	123,354
Gross unrealized gains	1,142	605
Gross unrealized losses	(286)	(390)
Estimated fair value	276,273	123,569
Commercial mortgage-backed securities [Member]
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	8,208	18,246
Gross unrealized gains	48	299
Gross unrealized losses	0	(2)
Estimated fair value	8,256	18,543
Fixed maturities [Member]
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	4,539,998	4,772,037
Gross unrealized gains	79,827	86,883
Gross unrealized losses	(16,291)	(35,053)
Estimated fair value	4,603,534	4,823,867
Short-term investments [Member]
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	725,230	273,444
Gross unrealized gains	57	70
Gross unrealized losses	(29)	0
Estimated fair value	725,258	273,514
Other investments level three [Member]
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Amortized cost	15,018	18,392
Gross unrealized gains	3,728	3,086
Gross unrealized losses	0	0
Estimated fair value	$ 18,746	$ 21,478