Fair Value Measurement - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011		Mar. 31, 2011
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	$ 157,034	[1]	$ 178,174	[1]	$ 197,714	[1]
Assets, Fair Value Disclosure	297,762	[1]	328,153	[1]	327,529	[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	246,600	[3]	250,541	[3]	242,524	[3]
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	157,034		178,174		197,714
Assets, Fair Value Disclosure	297,762		328,153		327,529
Liabilities, Fair Value Disclosure [Abstract]
Liabilities, Fair Value Disclosure	246,600		250,541		242,524
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	140,728		149,979		129,815
Liabilities, Fair Value Disclosure [Abstract]
Senior Notes Payable, Fair Value Disclosure	0		0		0
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		0		0
Liabilities, Fair Value Disclosure [Abstract]
Senior Notes Payable, Fair Value Disclosure	0		0		0
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		0		0
Liabilities, Fair Value Disclosure [Abstract]
Senior Notes Payable, Fair Value Disclosure	246,600		250,541		242,524
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	140,728		149,979		129,815
Liabilities, Fair Value Disclosure [Abstract]
Senior Notes Payable, Fair Value Disclosure	$ 246,600		$ 250,541		$ 242,524

[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.