CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 103,281	$ 615,191	$ 352,679	$ 967,307	$ 1,361,112
Other comprehensive income (loss):
Foreign currency translation	(5,018)	5,239	(8,061)	17,607
Unrealized gain (loss) on marketable debt securities	5,131	(48)	17,816	(233)
Other	2,801	(936)	17,418	(620)
Total other comprehensive income	2,914	4,255	27,173	16,754	$ 22,414
Comprehensive income	106,195	619,446	379,852	984,061
Comprehensive income attributable to noncontrolling interests	1,309	1,233	5,485	3,391
Comprehensive income attributable to common stockholders	$ 104,886	$ 618,213	$ 374,367	$ 980,670