Investments (Available-for-Sale Investments Classified by Contractual Maturity Date) (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Estimated Fair Value	$ 1,767,156,000	$ 1,812,512,000
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	34,400,000
Due after one year through five years, Amortized Cost	234,300,000
Due after five years through ten years, Amortized Cost	505,000,000
Due after ten years, Amortized Cost	531,200,000
Available for sale securities, Amortized Cost	1,734,953,000	1,753,931,000
Due in one year or less, Estimated Fair Value	34,400,000
Due after one year through five years, Estimated Fair Value	240,700,000
Due after five years through ten years, Estimated Fair Value	517,900,000
Due after ten years, Estimated Fair Value	534,900,000
Available for sale securities, Estimated Fair Value	1,767,200,000	1,812,500,000
Asset-backed Securities [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	430,100,000	499,700,000
Available for sale securities, Estimated Fair Value	$ 439,300,000	$ 519,100,000