Investments (Available for Sale Current and Noncurrent Investments Classified by Contractual Maturity Date) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Estimated Fair Value
Carrying Value, Current	$ 1,791,060,000	$ 1,567,020,000
Carrying value, Noncurrent	4,570,000	59,768,000
Current [Member]
Amortized Cost
Due in one year or less	69,400,000
Due after one year through five years	357,500,000
Due after five years through ten years	486,300,000
Due after ten years	427,000,000
Asset-backed securities	437,200,000
Amortized Cost	1,777,404,000	1,602,456,000
Estimated Fair Value
Due in one year or less	69,600,000
Due after one year through five years	357,800,000
Due after five years through ten years	489,600,000
Due after ten years	436,100,000
Asset-backed securities	437,900,000
Carrying Value, Current	1,791,000,000	1,567,000,000
Noncurrent [Member]
Amortized Cost
Due after one year through five years	1,300,000
Due after five years through ten years	3,400,000
Asset-backed securities	800,000
Amortized Cost	5,474,000	67,943,000
Estimated Fair Value
Due after one year through five years	1,100,000
Due after five years through ten years	2,900,000
Asset-backed securities	600,000
Carrying value, Noncurrent	$ 4,600,000	$ 59,800,000