Investments (Available-for-Sale Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Carrying Value, Noncurrent	$ 20,064	$ 4,570
Available for sale securities, Estimated Fair Value	2,221,278	1,791,060
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	42,300
Due after one year through five years, Amortized Cost	492,100
Due after five years through ten years, Amortized Cost	610,500
Due after ten years, Amortized Cost	509,300
Available for sale securities, Amortized Cost	2,210,387	1,777,404
Due in one year or less, Estimated Fair Value	42,400
Due after one year through five years, Estimated Fair Value	492,300
Due after five years through ten years, Estimated Fair Value	612,400
Due after ten years, Estimated Fair Value	516,700
Available for sale securities, Estimated Fair Value	2,221,300	1,791,000
Current [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	556,200	437,200
Available for sale securities, Estimated Fair Value	557,500	437,900
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Due after one year through five years, Amortized Cost	10,800
Due after five years through ten years, Amortized Cost	12,500
Available for sale securities, Amortized Cost	23,788	5,474
Carrying Value, Noncurrent	20,100	4,600
Due after one year through five years, Estimated Fair Value	9,000
Due after five years through ten years, Estimated Fair Value	10,700
Noncurrent [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	500	800
Carrying Value, Noncurrent	$ 400	$ 600