Investments (Available-for-Sale Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Carrying Value, Noncurrent	$ 6,544	$ 4,570
Available for sale securities, Estimated Fair Value	2,371,540	1,791,060
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	41,700
Due after one year through five years, Amortized Cost	538,300
Due after five years through ten years, Amortized Cost	642,800
Due after ten years, Amortized Cost	528,300
Available for sale securities, Amortized Cost	2,374,767	1,777,404
Due in one year or less, Estimated Fair Value	41,800
Due after one year through five years, Estimated Fair Value	538,100
Due after five years through ten years, Estimated Fair Value	640,900
Due after ten years, Estimated Fair Value	528,300
Available for sale securities, Estimated Fair Value	2,371,500	1,791,000
Current [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	623,700	437,200
Available for sale securities, Estimated Fair Value	622,400	437,900
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Due after five years through ten years, Amortized Cost	7,000
Available for sale securities, Amortized Cost	7,637	5,474
Carrying Value, Noncurrent	6,500	4,600
Due after five years through ten years, Estimated Fair Value	6,100
Noncurrent [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	600	800
Carrying Value, Noncurrent	$ 400	$ 600