Investments (Fair Value of Investments Available-for-Sale) (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Carrying Value, Current	$ 1,767,156,000	$ 1,812,512,000
Carrying Value, Noncurrent	0	0
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,734,953,000	1,753,931,000
Gross Unrealized Holding Gains	38,800,000	61,300,000
Gross Unrealized Holding Losses	(6,600,000)	(2,700,000)
Carrying Value, Current	1,767,200,000	1,812,500,000
Asset-Backed Securities [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	430,100,000	499,700,000
Gross Unrealized Holding Gains	9,900,000	19,600,000
Gross Unrealized Holding Losses	(700,000)	(200,000)
Carrying Value, Current	439,300,000	519,100,000
U.S. Government and Agencies [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26,400,000	25,900,000
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Carrying Value, Current	26,400,000	25,900,000
Obligations of States and Other Political Subdivisions [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	847,300,000	819,900,000
Gross Unrealized Holding Gains	18,600,000	24,200,000
Gross Unrealized Holding Losses	(5,100,000)	(2,000,000)
Carrying Value, Current	860,800,000	842,100,000
Corporate Debt Securities [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	431,200,000	408,400,000
Gross Unrealized Holding Gains	10,300,000	17,500,000
Gross Unrealized Holding Losses	(800,000)	(500,000)
Carrying Value, Current	$ 440,700,000	$ 425,400,000