Investments (Unrealized Losses On Investments) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value	$ 1,200,816	$ 983,887
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(1,835)	(2,747)
U.S. Government and agency obligations [Member]
Fair Value	214,612	50,604
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(73)	(23)
U.S. and foreign corporate Debt Securities [Member]
Fair Value	500,126	539,228
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(819)	(2,036)
Foreign governments and multi-national agency obligations [Member]
Fair Value	31,175	43,026
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(29)	(87)
Municipal Obligations [Member]
Fair Value	407,195	298,187
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(756)	(406)
Asset-Backed Securities [Member]
Fair Value	9,399	20,025
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(10)	(23)
Mortgage-Backed Securities [Member]
Fair Value	38,309	32,817
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	$ (148)	$ (172)