Cash Equivalents and Investments (Summary of Cash and Available-For-Sale Securities) (Detail) - USD ($) $ in Thousands	Mar. 31, 2016		Dec. 31, 2015		Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	$ 291,313		$ 219,682
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	643		670
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	0		0
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	291,956		220,352
Cash and Cash Equivalents	290,650	[1],[2]	218,883	[3]	$ 416,668	$ 291,377
Short-Term Marketable Securities	1,306	[1]	1,469	[3]
Cash [Member]
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	91,104		124,082
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0		0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	0		0
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	91,104		124,082
Cash and Cash Equivalents	91,104		124,082
Short-Term Marketable Securities	0		0
Money Market Funds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	199,546		94,801
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0		0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	0		0
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	199,546		94,801
Cash and Cash Equivalents	199,546		94,801
Short-Term Marketable Securities	0		0
Equity Securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	663		799
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	643		670
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	0		0
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	1,306		1,469
Cash and Cash Equivalents	0		0
Short-Term Marketable Securities	$ 1,306		$ 1,469

[1] unaudited

[2] unaudited

[3] Note 1