Investments - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule of Investments [Line Items]
Sale of stock			$ 91,900
Gain on warrant derivative			0	$ 76,699
Income tax benefit		$ 2,700		5,400
Equity Securities [Member]
Schedule of Investments [Line Items]
Net gains (losses) from changes in fair values of equity securities	$ (23,800)		62,900
Equity securities, unrealized gain			34,600
Equity Securities [Member]
Schedule of Investments [Line Items]
Equity securities	$ 169,400		$ 169,400
Equity securities, fair value					$ 188,400
Immunomedics [Member]
Schedule of Investments [Line Items]
Gain on warrant derivative		$ 78,700		$ 76,700