Net Investment Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Income (Losses) from Investment Funds	$ 117,831	$ 114,729	$ 439,240	$ 411,866
Investment expense	(1,812)	(666)	(4,352)	(2,605)
Net investment income	116,019	114,063	434,888	409,261
Fixed Maturity Securities, Including Cash [Member]
Income (Losses) from Investment Funds	121,454	120,252	360,577	365,365
Investment Funds [Member]
Income (Losses) from Investment Funds	(13,118)	(10,820)	50,124	21,069
Arbitrage Trading Account and Funds [Member]
Income (Losses) from Investment Funds	2,397	(346)	8,938	10,696
Equity Securities Available For Sale [Member]
Income (Losses) from Investment Funds	4,001	2,795	10,980	9,544
Real Estate [Member]
Income (Losses) from Investment Funds	$ 3,097	$ 2,848	$ 8,621	$ 5,192