Investments (Fair Value of Investments Available-For-Sale) (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Carrying Value, Current	$ 1,812,512,000	$ 1,557,997,000
Carrying Value, Noncurrent	0	2,147,000
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,753,931,000	1,528,091,000
Gross Unrealized Holding Gains	61,300,000	34,400,000
Gross Unrealized Holding Losses	(2,700,000)	(4,500,000)
Carrying Value, Current	1,812,500,000	1,558,000,000
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	2,450,000
Asset-backed securities [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	499,700,000	611,900,000
Gross Unrealized Holding Gains	19,600,000	10,600,000
Gross Unrealized Holding Losses	(200,000)	(200,000)
Carrying Value, Current	519,100,000	622,300,000
U.S. government and agencies [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	25,900,000	32,500,000
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Carrying Value, Current	25,900,000	32,500,000
Obligations of states and other political subdivisions [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	819,900,000	498,700,000
Gross Unrealized Holding Gains	24,200,000	19,500,000
Gross Unrealized Holding Losses	(2,000,000)	(100,000)
Carrying Value, Current	842,100,000	518,100,000
Corporate debt securities [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	408,400,000	385,000,000
Gross Unrealized Holding Gains	17,500,000	4,300,000
Gross Unrealized Holding Losses	(500,000)	(4,200,000)
Carrying Value, Current	425,400,000	385,100,000
Corporate debt securities [Member] | Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		2,400,000
Gross Unrealized Holding Gains		0
Gross Unrealized Holding Losses		(300,000)
Carrying Value, Noncurrent		$ 2,100,000