Investments (Fair Value of Investments Available-for-Sale) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Carrying Value, Current	$ 2,520,429,000	$ 1,791,060,000
Carrying Value, Noncurrent	6,328,000	4,570,000
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,490,857,000	1,777,404,000
Gross Unrealized Holding Gains	34,400,000	22,500,000
Gross Unrealized Holding Losses	(4,800,000)	(8,900,000)
Carrying Value, Current	2,520,400,000	1,791,000,000
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,471,000	5,474,000
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	(1,100,000)	(900,000)
Carrying Value, Noncurrent	6,300,000	4,600,000
Asset-Backed Securities [Member] \| Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	692,400,000	437,200,000
Gross Unrealized Holding Gains	5,100,000	2,600,000
Gross Unrealized Holding Losses	(1,000,000)	(1,900,000)
Carrying Value, Current	696,500,000	437,900,000
Asset-Backed Securities [Member] \| Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	800,000	800,000
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	(200,000)	(200,000)
Carrying Value, Noncurrent	600,000	600,000
U.S. Government and Agencies [Member] \| Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,700,000	36,500,000
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Carrying Value, Current	29,700,000	36,500,000
Obligations of States and Other Political Subdivisions [Member] \| Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	965,300,000	716,700,000
Gross Unrealized Holding Gains	20,800,000	17,200,000
Gross Unrealized Holding Losses	(2,300,000)	(1,700,000)
Carrying Value, Current	983,800,000	732,200,000
Corporate Debt Securities [Member] \| Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	803,400,000	587,000,000
Gross Unrealized Holding Gains	8,500,000	2,700,000
Gross Unrealized Holding Losses	(1,500,000)	(5,300,000)
Carrying Value, Current	810,400,000	584,400,000
Corporate Debt Securities [Member] \| Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,600,000	4,700,000
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	(900,000)	(700,000)
Carrying Value, Noncurrent	$ 5,700,000	$ 4,000,000

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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-Name Accounting Standards Codification

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-SubTopic 10

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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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Investments in debt and equity securities which are categorized neither as held-to-maturity nor trading and which are intended to be sold or mature more than one year from the balance sheet date or operating cycle, if longer. Such securities are reported at fair value; unrealized gains (losses) related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain (loss) of an Available-for-sale security that is designated as being hedged in a fair value hedge is recognized in earnings during the period of the hedge, as are other than temporary declines in fair value below the cost basis for investments in equity securities and debt securities that an entity intends to sell or it is more likely than not that it will be required to sell before the recovery of its amortized cost basis. Other than temporary declines in fair value below the cost basis for debt securities categorized as Available-for-sale that an entity does not intend to sell and for which it is not more likely than not that the entity will be required to sell before the recovery of its amortized cost basis are bifurcated into credit losses and losses related to all other factors. Other than temporary declines in fair value below cost basis related to credit losses are recognized in earnings, and losses related to all other factors are recognized in other comprehensive income.

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