Investments (Available-for-Sale Investments Classified by Contractual Maturity Date) (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Noncurrent	$ 52,637,000	$ 0
Available for sale securities, Estimated Fair Value	1,580,032,000	1,812,512,000
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	42,500,000
Due after one year through five years, Amortized Cost	281,600,000
Due after five years through ten years, Amortized Cost	442,800,000
Due after ten years, Amortized Cost	434,100,000
Available for sale securities, Amortized Cost	1,611,091,000	1,753,931,000
Due in one year or less, Estimated Fair Value	42,800,000
Due after one year through five years, Estimated Fair Value	284,100,000
Due after five years through ten years, Estimated Fair Value	434,900,000
Due after ten years, Estimated Fair Value	410,800,000
Available for sale securities, Estimated Fair Value	1,580,000,000	1,812,500,000
Current [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	410,100,000	499,700,000
Available for sale securities, Estimated Fair Value	407,400,000	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	9,500,000
Due after ten years, Amortized Cost	48,800,000
Available for sale securities, Amortized Cost	60,080,000	0
Available-for-sale Securities, Noncurrent	52,600,000
Due after one year through five years, Estimated Fair Value	800,000
Due after five years through ten years, Estimated Fair Value	8,300,000
Due after ten years, Estimated Fair Value	42,800,000
Noncurrent [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	800,000
Available-for-sale Securities, Noncurrent	$ 700,000