Fair Value Measurement - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012
Liabilities, Fair Value Disclosure [Abstract]
Line of Credit Facility, Amount Outstanding	$ 70,000		$ 70,000		$ 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Assets, Fair Value Disclosure	165,844		206,542		162,031
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Assets, Fair Value Disclosure	0		0		0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Assets, Fair Value Disclosure	0		0		0
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Assets, Fair Value Disclosure	165,844		206,542		162,031
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Inputs, Level 3 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Senior Notes Payable, Fair Value Disclosure	242,071	[1]	243,118	[1]	246,600	[1]
Line of Credit Facility, Amount Outstanding	70,568	[1]	70,444	[1]	0	[1]
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	160,845		201,542		157,034
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0		0		0
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0		0		0
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	160,845		201,542		157,034
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	4,999		5,000		4,997
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0		0		0
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0		0		0
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	$ 4,999		$ 5,000		$ 4,997

[1] 2The fair values of the senior notes payable and Credit Agreement loan are required to be disclosed only, and were based on borrowing rates available to us for long-term loans with similar terms, average maturities, and credit risk.