Investments (Fair Value of Investments Available-for-Sale) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Carrying Value, Current	$ 2,371,540	$ 1,791,060
Carrying Value, Noncurrent	6,544	4,570
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,374,767	1,777,404
Gross Unrealized Holding Gains	14,900	22,500
Gross Unrealized Holding Losses	(18,200)	(8,900)
Carrying Value, Current	2,371,500	1,791,000
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,637	5,474
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	(1,100)	(900)
Carrying Value, Noncurrent	6,500	4,600
Asset-Backed Securities [Member] \| Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	623,700	437,200
Gross Unrealized Holding Gains	2,300	2,600
Gross Unrealized Holding Losses	(3,600)	(1,900)
Carrying Value, Current	622,400	437,900
Asset-Backed Securities [Member] \| Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	600	800
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	(200)	(200)
Carrying Value, Noncurrent	400	600
U.S. Government and Agencies [Member] \| Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	28,200	36,500
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Carrying Value, Current	28,200	36,500
Obligations of States and Other Political Subdivisions [Member] \| Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	979,000	716,700
Gross Unrealized Holding Gains	10,000	17,200
Gross Unrealized Holding Losses	(9,000)	(1,700)
Carrying Value, Current	980,000	732,200
Obligations of States and Other Political Subdivisions [Member] \| Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,700
Gross Unrealized Holding Gains	0
Gross Unrealized Holding Losses	(200)
Carrying Value, Noncurrent	1,500
Corporate Debt Securities [Member] \| Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	743,900	587,000
Gross Unrealized Holding Gains	2,600	2,700
Gross Unrealized Holding Losses	(5,600)	(5,300)
Carrying Value, Current	740,900	584,400
Corporate Debt Securities [Member] \| Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,300	4,700
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	(700)	(700)
Carrying Value, Noncurrent	$ 4,600	$ 4,000

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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+ References

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-Publisher FASB

-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.

+ References

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

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