Investments (Fair Value of Investments Available-For-Sale) (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Current	$ 1,567,020,000	$ 1,812,512,000
Noncurrent	59,768,000	0
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,602,456,000	1,753,931,000
Gross Unrealized Holding Gains	12,900,000	61,300,000
Gross Unrealized Holding Losses	(48,400,000)	(2,700,000)
Current	1,567,000,000	1,812,500,000
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	67,943,000	0
Gross Unrealized Holding Gains	0
Gross Unrealized Holding Losses	(8,100,000)
Noncurrent	59,800,000
Asset-backed securities [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	394,700,000	499,700,000
Gross Unrealized Holding Gains	3,400,000	19,600,000
Gross Unrealized Holding Losses	(8,700,000)	(200,000)
Current	389,400,000	519,100,000
Asset-backed securities [Member] | Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,300,000
Gross Unrealized Holding Gains	0
Gross Unrealized Holding Losses	(200,000)
Noncurrent	1,100,000
U.S. government and agencies [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,700,000	25,900,000
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Current	23,700,000	25,900,000
Obligations of states and other political subdivisions [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	734,300,000	819,900,000
Gross Unrealized Holding Gains	5,900,000	24,200,000
Gross Unrealized Holding Losses	(30,300,000)	(2,000,000)
Current	709,900,000	842,100,000
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)
Noncurrent	47,100,000
Corporate debt securities [Member] | Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	449,800,000	408,400,000
Gross Unrealized Holding Gains	3,600,000	17,500,000
Gross Unrealized Holding Losses	(9,400,000)	(500,000)
Current	444,000,000	425,400,000
Corporate debt securities [Member] | Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,200,000
Gross Unrealized Holding Gains	0
Gross Unrealized Holding Losses	(1,600,000)
Noncurrent	$ 11,600,000