Investments (Fair Value of Investments Available-for-Sale) (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Carrying Value, Current	$ 1,685,009,000	$ 1,567,020,000
Carrying Value, Noncurrent	4,255,000	59,768,000
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,699,933,000	1,602,456,000
Gross Unrealized Holding Gains	14,300,000	12,900,000
Gross Unrealized Holding Losses	(29,200,000)	(48,400,000)
Carrying Value, Current	1,685,000,000	1,567,000,000
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,861,000	67,943,000
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	(600,000)	(8,100,000)
Carrying Value, Noncurrent	4,300,000	59,800,000
Asset-Backed Securities [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	415,800,000	394,700,000
Gross Unrealized Holding Gains	3,500,000	3,400,000
Gross Unrealized Holding Losses	(6,600,000)	(8,700,000)
Carrying Value, Current	412,700,000	389,400,000
Asset-Backed Securities [Member] | Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	800,000	1,300,000
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	(100,000)	(200,000)
Carrying Value, Noncurrent	700,000	1,100,000
U.S. Government and Agencies [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,700,000	23,700,000
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Carrying Value, Current	23,700,000	23,700,000
Obligations of States and Other Political Subdivisions [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	758,700,000	734,300,000
Gross Unrealized Holding Gains	7,100,000	5,900,000
Gross Unrealized Holding Losses	(17,500,000)	(30,300,000)
Carrying Value, Current	748,300,000	709,900,000
Obligations of States and Other Political Subdivisions [Member] | Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		53,400,000
Gross Unrealized Holding Gains		0
Gross Unrealized Holding Losses		(6,300,000)
Carrying Value, Noncurrent		47,100,000
Corporate Debt Securities [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	501,700,000	449,800,000
Gross Unrealized Holding Gains	3,700,000	3,600,000
Gross Unrealized Holding Losses	(5,100,000)	(9,400,000)
Carrying Value, Current	500,300,000	444,000,000
Corporate Debt Securities [Member] | Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,100,000	13,200,000
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	(500,000)	(1,600,000)
Carrying Value, Noncurrent	$ 3,600,000	$ 11,600,000