Investments (Available-for-Sale Investments Classified by Contractual Maturity Date) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Carrying Value, Noncurrent	$ 6,328,000	$ 4,570,000
Available for sale securities, Estimated Fair Value	2,520,429,000	1,791,060,000
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	56,600,000
Due after one year through five years, Amortized Cost	599,900,000
Due after five years through ten years, Amortized Cost	628,700,000
Due after ten years, Amortized Cost	513,200,000
Available for sale securities, Amortized Cost	2,490,857,000	1,777,404,000
Due in one year or less, Estimated Fair Value	56,700,000
Due after one year through five years, Estimated Fair Value	603,800,000
Due after five years through ten years, Estimated Fair Value	639,700,000
Due after ten years, Estimated Fair Value	523,700,000
Available for sale securities, Estimated Fair Value	2,520,400,000	1,791,000,000
Current [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	692,400,000	437,200,000
Available for sale securities, Estimated Fair Value	696,500,000	437,900,000
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Due after one year through five years, Amortized Cost	200,000
Due after five years through ten years, Amortized Cost	6,400,000
Available for sale securities, Amortized Cost	7,471,000	5,474,000
Carrying Value, Noncurrent	6,300,000	4,600,000
Due after one year through five years, Estimated Fair Value	200,000
Due after five years through ten years, Estimated Fair Value	5,500,000
Noncurrent [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	800,000	800,000
Carrying Value, Noncurrent	$ 600,000	$ 600,000