Investments (Contractual Maturity Of Investments) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Total investments, Amortized Cost Basis	$ 3,265,148	$ 2,554,426
Fixed Income Securities [Member]
Due within one year, Amortized Cost Basis	1,275,888
Due after 1 year through 5 years, Amortized Cost Basis	1,929,232
Due after 5 years, Amortized Cost Basis	60,028
Total investments, Amortized Cost Basis	3,265,148
Due within one year, Aggregate Fair Value	1,276,571
Due after 1 year through 5 years, Aggregate Fair Value	1,933,370
Due after 5 years, Aggregate Fair Value	60,028
Total investments, Aggregate Fair Value	$ 3,269,969