Marketable securities - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Realized gain (loss) on available-for-sale securities	$ 0	$ 0
Reclassification out of accumulated other comprehensive income (loss)	0	$ 0
Unrealized Loss on Securities	918,900,000		$ 704,100,000
Unrealized loss on securities, more than twelve months	142,700,000		134,400,000
Investments with other-than-temporary impairment	$ 0		$ 0