Fair Value Measurement - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011		Jun. 30, 2011
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	$ 167,427	[1]	$ 178,174	[1]	$ 163,058	[1]
Assets, Fair Value Disclosure	256,666	[1]	328,153	[1]	291,321	[1]
Liabilities, Fair Value Disclosure [Abstract]
Liabilities, Fair Value Disclosure	0	[1]	0	[1]	0	[1]
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	0	[2]	0	[2]	0	[2]
Assets, Fair Value Disclosure	0	[2]	0	[2]	0	[2]
Liabilities, Fair Value Disclosure [Abstract]
Liabilities, Fair Value Disclosure	0	[2]	0	[2]	0	[2]
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	0	[3]	0	[3]	0	[3]
Assets, Fair Value Disclosure	0	[3]	0	[3]	0	[3]
Liabilities, Fair Value Disclosure [Abstract]
Liabilities, Fair Value Disclosure	239,443	[3]	250,541	[3]	239,641	[3]
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	167,427		178,174		163,058
Assets, Fair Value Disclosure	256,666		328,153		291,321
Liabilities, Fair Value Disclosure [Abstract]
Liabilities, Fair Value Disclosure	239,443		250,541		239,641
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Held-to-maturity Securities, Fair Value Disclosure	89,239	[1]	149,979	[1]	128,263	[1]
Liabilities, Fair Value Disclosure [Abstract]
Senior Notes Payable, Fair Value Disclosure	0	[1]	0	[1]	0	[1]
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Held-to-maturity Securities, Fair Value Disclosure	0	[2]	0	[2]	0	[2]
Liabilities, Fair Value Disclosure [Abstract]
Senior Notes Payable, Fair Value Disclosure	0	[2]	0	[2]	0	[2]
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Held-to-maturity Securities, Fair Value Disclosure	0	[3]	0	[3]	0	[3]
Liabilities, Fair Value Disclosure [Abstract]
Senior Notes Payable, Fair Value Disclosure	239,443	[3]	250,541	[3]	239,641	[3]
Estimate of Fair Value, Fair Value Disclosure [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Held-to-maturity Securities, Fair Value Disclosure	89,239		149,979		128,263
Liabilities, Fair Value Disclosure [Abstract]
Senior Notes Payable, Fair Value Disclosure	$ 239,443		$ 250,541		$ 239,641

[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.