Investments in Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Current	$ 249,754	$ 248,954
Total Investments, Cost Basis	256,787	257,500
Total Investments, Accumulated Gross Unrealized Gains	1,973	4,125
Total Investments, Accumulated Gross Unrealized Losses	(5,975)	(7,526)
Total Investments	252,785	254,099
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total Investments, Current	249,754	248,954
Other Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total Investments, Noncurrent	3,031	5,145
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Cost Basis	252,877	253,590
Debt Securities, Accumulated Gross Unrealized Gains	0	0
Debt Securities, Accumulated Gross Unrealized Losses	(3,123)	(4,636)
Debt Securities	249,754	248,954
Corporate Debt Securities [Member] | Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Current	249,754	248,954
Corporate Debt Securities [Member] | Other Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Noncurrent	0	0
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, Cost Basis	3,910	3,910
Equity Securities, Accumulated Gross Unrealized Gains	1,973	4,125
Equity Securities, Accumulated Gross Unrealized Losses	(2,852)	(2,890)
Equity securities	3,031	5,145
Equity Securities [Member] | Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, Current	0	0
Equity Securities [Member] | Other Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, Noncurrent	$ 3,031	$ 5,145