Investments (Available-for-Sale Investments Classified by Contractual Maturity Date) (Details) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Noncurrent	$ 18,332,000	$ 0
Available for sale securities, Estimated Fair Value	1,632,466,000	1,812,512,000
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	41,900,000
Due after one year through five years, Amortized Cost	262,600,000
Due after five years through ten years, Amortized Cost	466,600,000
Due after ten years, Amortized Cost	485,900,000
Available for sale securities, Amortized Cost	1,669,326,000	1,753,931,000
Due in one year or less, Estimated Fair Value	42,100,000
Due after one year through five years, Estimated Fair Value	263,800,000
Due after five years through ten years, Estimated Fair Value	456,600,000
Due after ten years, Estimated Fair Value	461,500,000
Available for sale securities, Estimated Fair Value	1,632,500,000	1,812,500,000
Current [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	412,300,000	499,700,000
Available for sale securities, Estimated Fair Value	408,500,000	519,100,000
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Due after one year through five years, Amortized Cost	1,000,000
Due after five years through ten years, Amortized Cost	800,000
Due after ten years, Amortized Cost	17,700,000
Available for sale securities, Amortized Cost	20,679,000	0
Available-for-sale Securities, Noncurrent	18,300,000
Due after one year through five years, Estimated Fair Value	800,000
Due after five years through ten years, Estimated Fair Value	700,000
Due after ten years, Estimated Fair Value	15,700,000
Noncurrent [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	1,200,000
Available-for-sale Securities, Noncurrent	$ 1,100,000