Cash Equivalents and Investments (Summary of Cash and Available-For-Sale Securities) (Detail) - USD ($) $ in Thousands	Jun. 30, 2017		Dec. 31, 2016		Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	$ 360,323		$ 167,141
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	360,323		167,141
Cash and cash equivalents	349,428	[1],[2]	147,154	[3]	$ 115,854	$ 218,883
Short-Term Marketable Securities	10,895	[1]	19,987	[3]
Cash [Member]
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	340,246		147,150
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	340,246		147,150
Cash and cash equivalents	340,246		147,150
Short-Term Marketable Securities	0		0
Money Market Funds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	9,182		4
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	9,182		4
Cash and cash equivalents	9,182		4
Short-Term Marketable Securities	0		0
Commercial Paper [Member]
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	10,895		19,987
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	10,895		19,987
Cash and cash equivalents	0		0
Short-Term Marketable Securities	$ 10,895		$ 19,987

[1] unaudited

[2] unaudited

[3] Note 1