Investments (Investing In Fixed Income Securities) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Cost or Amortized Cost	$ 3,265,148	$ 2,554,426
Unrealized Gains	6,658	4,802
Unrealized Losses	(1,837)	(2,778)
Short-term investments	3,269,969	2,556,450
U.S. Government and agency obligations [Member]
Cost or Amortized Cost	475,556	516,795
Unrealized Gains	1,220	1,842
Unrealized Losses	(73)	(23)
Total investments, Aggregate Fair Value	476,703	518,614
U.S. and foreign corporate Debt Securities [Member]
Cost or Amortized Cost	1,488,144	1,134,009
Unrealized Gains	2,800	1,404
Unrealized Losses	(821)	(2,036)
Total investments, Aggregate Fair Value	1,490,123	1,133,377
Foreign governments and multi-national agency obligations [Member]
Cost or Amortized Cost	39,138	58,455
Unrealized Gains	14	30
Unrealized Losses	(29)	(87)
Total investments, Aggregate Fair Value	39,123	58,398
Municipal Obligations [Member]
Cost or Amortized Cost	1,160,540	768,282
Unrealized Gains	2,442	1,396
Unrealized Losses	(756)	(437)
Total investments, Aggregate Fair Value	1,162,226	769,241
Asset-Backed Securities [Member]
Cost or Amortized Cost	36,648	27,107
Unrealized Gains	53	2
Unrealized Losses	(10)	(23)
Total investments, Aggregate Fair Value	36,691	27,086
Mortgage-Backed Securities [Member]
Cost or Amortized Cost	65,122	49,778
Unrealized Gains	129	128
Unrealized Losses	(148)	(172)
Total investments, Aggregate Fair Value	$ 65,103	$ 49,734