Investments (Available for Sale Current and Noncurrent Investments Classified by Contractual Maturity Date) (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Carrying Value, Current	$ 1,812,512,000	$ 1,557,997,000
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	34,200,000
Due after one year through five years, Amortized Cost	211,600,000
Due after five years through ten years, Amortized Cost	485,300,000
Due after ten years, Amortized Cost	523,100,000
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	499,700,000
Available-for-sale Securities, Amortized Cost Basis	1,753,931,000	1,528,091,000
Due in one year or less, Estimated Fair Value	34,300,000
Due after one year through five years, Estimated Fair Value	218,700,000
Due after five years through ten years, Estimated Fair Value	504,500,000
Due after ten years, Estimated Fair Value	535,900,000
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value	519,100,000
Carrying Value, Current	1,812,500,000	1,558,000,000
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 0	$ 2,450,000