Investments (Available-for-Sale Investments Classified by Contractual Maturity Date) (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Noncurrent	$ 4,255,000	$ 59,768,000
Available for sale securities, Estimated Fair Value	1,685,009,000	1,567,020,000
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	56,000,000
Due after one year through five years, Amortized Cost	330,400,000
Due after five years through ten years, Amortized Cost	450,500,000
Due after ten years, Amortized Cost	447,200,000
Available for sale securities, Amortized Cost	1,699,933,000	1,602,456,000
Due in one year or less, Estimated Fair Value	56,300,000
Due after one year through five years, Estimated Fair Value	333,200,000
Due after five years through ten years, Estimated Fair Value	449,300,000
Due after ten years, Estimated Fair Value	433,500,000
Available for sale securities, Estimated Fair Value	1,685,000,000	1,567,000,000
Current [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	415,800,000	394,700,000
Available for sale securities, Estimated Fair Value	412,700,000	389,400,000
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Due after five years through ten years, Amortized Cost	2,100,000
Due after ten years, Amortized Cost	2,000,000
Available for sale securities, Amortized Cost	4,861,000	67,943,000
Available-for-sale Securities, Noncurrent	4,300,000	59,800,000
Due after five years through ten years, Estimated Fair Value	1,900,000
Due after ten years, Estimated Fair Value	1,700,000
Noncurrent [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	800,000	1,300,000
Available-for-sale Securities, Noncurrent	$ 700,000	$ 1,100,000