Investments (Available-for-Sale Investments Classified by Contractual Maturity Date) (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Noncurrent	$ 652,000	$ 59,768,000
Available for sale securities, Estimated Fair Value	1,668,943,000	1,567,020,000
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	58,000,000
Due after one year through five years, Amortized Cost	325,200,000
Due after five years through ten years, Amortized Cost	417,300,000
Due after ten years, Amortized Cost	440,500,000
Available for sale securities, Amortized Cost	1,662,725,000	1,602,456,000
Due in one year or less, Estimated Fair Value	58,200,000
Due after one year through five years, Estimated Fair Value	328,600,000
Due after five years through ten years, Estimated Fair Value	422,300,000
Due after ten years, Estimated Fair Value	437,700,000
Available for sale securities, Estimated Fair Value	1,668,900,000	1,567,000,000
Current [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	421,700,000	394,700,000
Available for sale securities, Estimated Fair Value	422,100,000	389,400,000
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	808,000	67,943,000
Available-for-sale Securities, Noncurrent	700,000	59,800,000
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