Investments (Available-for-Sale Current Investments Classified by Contractual Maturity Date) (Details) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Estimated Fair Value	$ 1,671,678,000	$ 1,557,997,000
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	33,000,000
Due after one year through five years, Amortized Cost	190,400,000
Due after five years through ten years, Amortized Cost	406,400,000
Due after ten years, Amortized Cost	402,600,000
Available for sale securities, Amortized Cost	1,607,413,000	1,528,091,000
Due in one year or less, Estimated Fair Value	33,100,000
Due after one year through five years, Estimated Fair Value	197,200,000
Due after five years through ten years, Estimated Fair Value	427,200,000
Due after ten years, Estimated Fair Value	416,300,000
Available for sale securities, Estimated Fair Value	1,671,700,000	1,558,000,000
Current [Member] | Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	575,000,000	611,900,000
Available for sale securities, Estimated Fair Value	$ 597,900,000	$ 622,300,000