Summary of Fair Value Accumulated Other Comprehensive Income (Loss) and Intraperiod Tax Allocation in Available-for-Sale Marketable Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Intraperiod tax allocation recorded as a benefit from income taxes	$ (14,947)	$ (32,793)
Accumulated other comprehensive income (loss), net of tax	(44,394)	31,127	$ 4,055
Regulus Therapeutics Inc
Schedule of Available-for-sale Securities [Line Items]
Carrying value	11,935	11,935	12,449
Accumulated other comprehensive income (loss), before tax	39,484	82,648	33,003
Investment in equity securities of Regulus Therapeutics Inc., as reported	51,419	94,583	45,452
Accumulated other comprehensive income (loss), before tax	39,484	82,648	33,003
Intraperiod tax allocation recorded as a benefit from income taxes	(32,792)	(32,792)	(13,267)
Accumulated other comprehensive income (loss), net of tax	6,692	49,856	$ 19,736
Regulus Therapeutics Inc | Sales of Regulus Shares
Schedule of Available-for-sale Securities [Line Items]
Carrying value		(514)
Accumulated other comprehensive income (loss), before tax		(2,081)
Investment in equity securities of Regulus Therapeutics Inc., as reported		(2,595)
Accumulated other comprehensive income (loss), before tax		(2,081)
Accumulated other comprehensive income (loss), net of tax		(2,081)
Regulus Therapeutics Inc | All Other Activity
Schedule of Available-for-sale Securities [Line Items]
Accumulated other comprehensive income (loss), before tax	(43,164)	51,726
Investment in equity securities of Regulus Therapeutics Inc., as reported	(43,164)	51,726
Accumulated other comprehensive income (loss), before tax	(43,164)	51,726
Intraperiod tax allocation recorded as a benefit from income taxes		(19,525)
Accumulated other comprehensive income (loss), net of tax	$ (43,164)	$ 32,201