Investments (Fair Value of Investments Available-for-Sale) (Details) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Carrying Value, Current	$ 1,632,466,000	$ 1,812,512,000
Carrying Value, Noncurrent	18,332,000	0
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,669,326,000	1,753,931,000
Gross Unrealized Holding Gains	12,600,000	61,300,000
Gross Unrealized Holding Losses	(49,400,000)	(2,700,000)
Carrying Value, Current	1,632,500,000	1,812,500,000
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,679,000	0
Gross Unrealized Holding Gains	0
Gross Unrealized Holding Losses	(2,400,000)
Carrying Value, Noncurrent	18,300,000
Asset-Backed Securities [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	412,300,000	499,700,000
Gross Unrealized Holding Gains	3,800,000	19,600,000
Gross Unrealized Holding Losses	(7,600,000)	(200,000)
Carrying Value, Current	408,500,000	519,100,000
Asset-Backed Securities [Member] | Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,200,000
Gross Unrealized Holding Gains	0
Gross Unrealized Holding Losses	(100,000)
Carrying Value, Noncurrent	1,100,000
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	791,900,000	819,900,000
Gross Unrealized Holding Gains	6,300,000	24,200,000
Gross Unrealized Holding Losses	(30,600,000)	(2,000,000)
Carrying Value, Current	767,600,000	842,100,000
Obligations of States and Other Political Subdivisions [Member] | Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,200,000
Gross Unrealized Holding Gains	0
Gross Unrealized Holding Losses	(1,600,000)
Carrying Value, Noncurrent	12,600,000
Corporate Debt Securities [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	438,700,000	408,400,000
Gross Unrealized Holding Gains	2,500,000	17,500,000
Gross Unrealized Holding Losses	(11,200,000)	(500,000)
Carrying Value, Current	430,000,000	425,400,000
Corporate Debt Securities [Member] | Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,300,000
Gross Unrealized Holding Gains	0
Gross Unrealized Holding Losses	(700,000)
Carrying Value, Noncurrent	$ 4,600,000