Cash, Cash Equivalents, and Investments - Components of Cash, Cash Equivalents, and Investments (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Cash	$ 40,124,000	$ 34,986,000
Cash equivalents		5,098,000
Money market instruments	520,000	5,076,000
Cash and cash equivalents	40,644,000	45,160,000	38,103,000	33,203,000
Short-term investments	184,822,000	105,105,000
Limited partnership interest (accounted for using cost method)	1,956,000	3,740,000
Long-term investments	229,655,000	238,255,000
Total	455,121,000	388,520,000
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Short-term investments	109,040,000	46,001,000
Long-term investments	109,909,000	100,072,000
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Short-term investments	53,559,000	17,666,000
Long-term investments	21,820,000	34,710,000
Sovereign Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Short-term investments	11,448,000	3,986,000
Long-term investments	16,385,000	10,606,000
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Short-term investments	9,276,000	16,224,000
Long-term investments	5,919,000	17,846,000
Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Short-term investments	1,499,000	7,482,000
Long-term investments		29,441,000
Treasury Bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Short-term investments		5,997,000
Long-term investments	73,666,000	36,276,000
Covered Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Short-term investments		5,618,000
Long-term investments		5,564,000
Corporate Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Short-term investments		$ 2,131,000