Cash Equivalents and Investments (Summary of Cash and Available-For-Sale Securities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017		Dec. 31, 2016		Sep. 30, 2016	[2]	Dec. 31, 2015	[3]
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	$ 515,497		$ 167,141
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	516,494		167,141
Cash and cash equivalents	510,085	[1],[2]	147,154	[3]	$ 106,590		$ 218,883
Short-Term Marketable Securities	6,409	[1]	19,987	[3]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	997		0
Cash [Member]
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	495,380		147,150
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	495,380		147,150
Cash and cash equivalents	495,380		147,150
Short-Term Marketable Securities	0		0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0		0
Money Market Funds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	14,705		4
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	14,705		4
Cash and cash equivalents	14,705		4
Short-Term Marketable Securities	0		0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0		0
Commercial Paper [Member]
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	2,060		19,987
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	2,060		19,987
Cash and cash equivalents	0		0
Short-Term Marketable Securities	2,060		$ 19,987
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0
Equity Securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	3,352
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	4,349
Cash and cash equivalents	0
Short-Term Marketable Securities	4,349
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	$ 997

[1] unaudited

[2] unaudited

[3] Note 1