Fair Value Measurement - Carrying and Fair Value Amounts (Details) - USD ($) $ in Thousands		Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		$ 30,082	$ 62,024	$ 23,975
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		4,994
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity marketable securities		77,678	105,336	88,075
Fair Value, Inputs, Level 1 [Member] | Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity marketable securities		77,612	105,695	88,068
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		30,082	62,024	23,975
Fair Value, Inputs, Level 1 [Member] | Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		4,994
Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes payable	[1]	167,210	165,731	212,648
Credit Agreement loan	[1]	97,409	99,375	69,387
Fair Value, Inputs, Level 3 [Member] | Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes payable		160,000	160,000	200,000
Credit Agreement loan		97,500	100,000	70,000
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		0	0	0
Fair Value, Inputs, Level 3 [Member] | Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		0
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		0	$ 0	$ 0
Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		$ 0

[1] The fair values of the 2019 Notes and Credit Agreement (defined in Note 10) loan are based on borrowing rates available to us for long-term debt with similar terms, average maturities, and credit risk.