Investments (Available for Sale Current and Noncurrent Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Estimated Fair Value
Carrying Value, Current	$ 2,218,367	$ 1,791,060
Carrying value, Noncurrent	27,580	4,570
Current [Member]
Amortized Cost
Due in one year or less	51,500
Due after one year through five years	514,200
Due after five years through ten years	579,900
Due after ten years	503,700
Asset-backed securities	560,400
Amortized Cost	2,209,670	1,777,404
Estimated Fair Value
Due in one year or less	51,400
Due after one year through five years	511,700
Due after five years through ten years	582,100
Due after ten years	517,600
Asset-backed securities	555,600
Carrying Value, Current	2,218,400	1,791,000
Noncurrent [Member]
Amortized Cost
Due after one year through five years	15,800
Due after five years through ten years	17,200
Due after ten years	1,200
Amortized Cost	34,197	5,474
Estimated Fair Value
Due after one year through five years	13,100
Due after five years through ten years	13,400
Due after ten years	1,100
Carrying value, Noncurrent	$ 27,600	$ 4,600