Net Investment Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2012
Income (Losses) from Investment Funds	$ 162,691	$ 150,158	$ 297,137	$ 321,409
Investment expense	(1,441)	(1,086)	(1,939)	(2,540)
Net investment income	161,250	149,072	295,198	318,869
Fixed Maturity Securities, Including Cash [Member]
Income (Losses) from Investment Funds	119,835	124,172	245,113	239,123
Investment Funds [Member]
Income (Losses) from Investment Funds	35,619	15,502	31,889	63,242
Arbitrage Trading Account and Funds [Member]
Income (Losses) from Investment Funds	60	5,827	11,042	6,541
Equity Securities Available For Sale [Member]
Income (Losses) from Investment Funds	3,829	3,485	6,749	6,979
Real Estate [Member]
Income (Losses) from Investment Funds	$ 3,348	$ 1,172	$ 2,344	$ 5,524