Fair Value Measurements - Summary of Placement in the Fair Value Hierarchy of Financial Assets that are Measured at Fair Value on a Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Assets
Fair value of assets	$ 247,574	$ 330,086
US Government Debt Securities
Assets
Fair value of assets	148,748	178,299
Commercial Paper
Assets
Fair value of assets		7,997
Certificates of Deposit
Assets
Fair value of assets	1,020	2,265
Equity Securities
Assets
Fair value of assets	97,806	141,525
Quoted prices in active markets (level 1)
Assets
Fair value of assets	82,096	110,297
Quoted prices in active markets (level 1) | US Government Debt Securities
Assets
Fair value of assets	0	0
Quoted prices in active markets (level 1) | Commercial Paper
Assets
Fair value of assets		0
Quoted prices in active markets (level 1) | Certificates of Deposit
Assets
Fair value of assets	0	0
Quoted prices in active markets (level 1) | Equity Securities
Assets
Fair value of assets	82,096	110,297
Significant other observable inputs (level 2)
Assets
Fair value of assets	165,478	219,789
Significant other observable inputs (level 2) | US Government Debt Securities
Assets
Fair value of assets	148,748	178,299
Significant other observable inputs (level 2) | Commercial Paper
Assets
Fair value of assets		7,997
Significant other observable inputs (level 2) | Certificates of Deposit
Assets
Fair value of assets	1,020	2,265
Significant other observable inputs (level 2) | Equity Securities
Assets
Fair value of assets	15,710	31,228
Significant unobservable inputs (level 3)
Assets
Fair value of assets	0	0
Significant unobservable inputs (level 3) | US Government Debt Securities
Assets
Fair value of assets	0	0
Significant unobservable inputs (level 3) | Commercial Paper
Assets
Fair value of assets		0
Significant unobservable inputs (level 3) | Certificates of Deposit
Assets
Fair value of assets	0	0
Significant unobservable inputs (level 3) | Equity Securities
Assets
Fair value of assets	$ 0	$ 0