Fair Value Measurement - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) (USD $)	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]		Sep. 30, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]		Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]		Sep. 30, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]		Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]		Sep. 30, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]		Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Estimate of Fair Value, Fair Value Disclosure [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Estimate of Fair Value, Fair Value Disclosure [Member]	Sep. 30, 2011 Fair Value, Measurements, Recurring [Member] Estimate of Fair Value, Fair Value Disclosure [Member]	Sep. 30, 2012 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 1 [Member]		Dec. 31, 2011 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 1 [Member]		Sep. 30, 2011 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 1 [Member]		Sep. 30, 2012 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 2 [Member]		Dec. 31, 2011 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 2 [Member]		Sep. 30, 2011 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 2 [Member]		Sep. 30, 2012 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 3 [Member]		Dec. 31, 2011 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 3 [Member]		Sep. 30, 2011 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 3 [Member]		Sep. 30, 2012 Estimate of Fair Value, Fair Value Disclosure [Member]	Dec. 31, 2011 Estimate of Fair Value, Fair Value Disclosure [Member]	Sep. 30, 2011 Estimate of Fair Value, Fair Value Disclosure [Member]	Sep. 30, 2012 Money Market Funds [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]		Dec. 31, 2011 Money Market Funds [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]		Sep. 30, 2011 Money Market Funds [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]		Sep. 30, 2012 Money Market Funds [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]		Dec. 31, 2011 Money Market Funds [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]		Sep. 30, 2011 Money Market Funds [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]		Sep. 30, 2012 Money Market Funds [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]		Dec. 31, 2011 Money Market Funds [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]		Sep. 30, 2011 Money Market Funds [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]		Sep. 30, 2012 Money Market Funds [Member] Fair Value, Measurements, Recurring [Member] Estimate of Fair Value, Fair Value Disclosure [Member]	Dec. 31, 2011 Money Market Funds [Member] Fair Value, Measurements, Recurring [Member] Estimate of Fair Value, Fair Value Disclosure [Member]	Sep. 30, 2011 Money Market Funds [Member] Fair Value, Measurements, Recurring [Member] Estimate of Fair Value, Fair Value Disclosure [Member]	Sep. 30, 2012 Commercial paper [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]		Dec. 31, 2011 Commercial paper [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]		Sep. 30, 2011 Commercial paper [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]		Sep. 30, 2012 Commercial paper [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]		Dec. 31, 2011 Commercial paper [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]		Sep. 30, 2011 Commercial paper [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]		Sep. 30, 2012 Commercial paper [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]		Dec. 31, 2011 Commercial paper [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]		Sep. 30, 2011 Commercial paper [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]		Sep. 30, 2012 Commercial paper [Member] Fair Value, Measurements, Recurring [Member] Estimate of Fair Value, Fair Value Disclosure [Member]	Dec. 31, 2011 Commercial paper [Member] Fair Value, Measurements, Recurring [Member] Estimate of Fair Value, Fair Value Disclosure [Member]	Sep. 30, 2011 Commercial paper [Member] Fair Value, Measurements, Recurring [Member] Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cost-method Investments, Other than Temporary Impairment	$ 2,800,000
Assets, Fair Value Disclosure [Abstract]
Cash equivalents																																												185,586,000	[1]	178,174,000	[1]	149,966,000	[1]	0	[2]	0	[2]	0	[2]	0	[3]	0	[3]	0	[3]	185,586,000	178,174,000	149,966,000	14,495,000	[1]	4,999,000	[1]	4,997,000	[1]	0	[2]	0	[2]	0	[2]	0	[3]	0	[3]	0	[3]	14,495,000	4,999,000	4,997,000
Held-to-maturity Securities, Fair Value Disclosure																							85,133,000	[1]	149,979,000	[1]	137,194,000	[1]	0	[2]	0	[2]	0	[2]	0	[3]	0	[3]	0	[3]	85,133,000	149,979,000	137,194,000
Assets, Fair Value Disclosure		285,214,000	[1]	333,152,000	[1]	292,157,000	[1]	0	[2]	0	[2]	0	[2]	0	[3]	0	[3]	0	[3]	285,214,000	333,152,000	292,157,000
Liabilities, Fair Value Disclosure [Abstract]
Senior Notes Payable, Fair Value Disclosure																							0	[1]	0	[1]	0	[1]	0	[2]	0	[2]	0	[2]	243,219,000	[3]	250,541,000	[3]	242,698,000	[3]	243,219,000	250,541,000	242,698,000
Liabilities, Fair Value Disclosure		$ 0	[1]	$ 0	[1]	$ 0	[1]	$ 0	[2]	$ 0	[2]	$ 0	[2]	$ 243,219,000	[3]	$ 250,541,000	[3]	$ 242,698,000	[3]	$ 243,219,000	$ 250,541,000	$ 242,698,000

[1] Quoted prices in active markets for identical assets or liabilities.

[2] Observable inputs other than Level 1 prices such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

[3] Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.