Concentration of Credit Risk and Fair Value of Financial Assets and Liabilities Fair Value of Financial Assets and Liabilities (Details) (USD $)	9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	$ 756,842,000			$ 577,279,000
Commercial Paper	37,200,000			87,900,000
LONG-TERM DEBT	206,600,000			200,400,000
LONG-TERM DEBT	206,635,000
Long-term Debt, Fair Value	240,900,000			245,200,000
Other than Temporary Impairment Losses, Investments	6,669,000		0
Investment Owned, at Fair Value	15,100,000
Money Market Funds and Demand Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	518,352,000	[1]		261,899,000	[1]
Mutual and Exchange Traded Funds [Domain]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring				100,682,000
Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	188,060,000			150,868,000
US Government and Government Agencies and Authorities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	37,463,000			50,560,000
Corporate Bonds and Asset Backed Securities [Domain]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	12,967,000			13,270,000
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	520,118,000			362,581,000
Fair Value, Inputs, Level 1 [Member] | Money Market Funds and Demand Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	518,352,000			261,899,000
Fair Value, Inputs, Level 1 [Member] | Mutual and Exchange Traded Funds [Domain]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring				100,682,000
Fair Value, Inputs, Level 1 [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0			0
Fair Value, Inputs, Level 1 [Member] | US Government and Government Agencies and Authorities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0			0
Fair Value, Inputs, Level 1 [Member] | Corporate Bonds and Asset Backed Securities [Domain]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	1,766,000			0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	236,724,000			214,698,000
Fair Value, Inputs, Level 2 [Member] | Money Market Funds and Demand Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0			0
Fair Value, Inputs, Level 2 [Member] | Mutual and Exchange Traded Funds [Domain]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring				0
Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	188,060,000			150,868,000
Fair Value, Inputs, Level 2 [Member] | Corporate Bonds and Asset Backed Securities [Domain]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	11,201,000			13,270,000
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0			0
Fair Value, Inputs, Level 3 [Member] | Money Market Funds and Demand Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0			0
Fair Value, Inputs, Level 3 [Member] | Mutual and Exchange Traded Funds [Domain]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring				0
Fair Value, Inputs, Level 3 [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0			0
Fair Value, Inputs, Level 3 [Member] | US Government and Government Agencies and Authorities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	0			0
Fair Value, Inputs, Level 3 [Member] | Corporate Bonds and Asset Backed Securities [Domain]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Recurring	$ 0			$ 0

[1] Includes $87.9 million of commercial paper that is included within cash and cash equivalents.