CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 615,191	$ 150,184	$ 967,307	$ 442,658	$ 651,490
Other comprehensive income (loss):
Foreign currency translation	5,239	(6,421)	17,607	(39,804)
Unrealized (loss) gain on government-sponsored pooled loan investments	(48)	(92)	(233)	158
Other	(936)	1,094	(620)	(2,403)
Total other comprehensive income (loss)	4,255	(5,419)	16,754	(42,049)	$ (49,969)
Comprehensive income	619,446	144,765	984,061	400,609
Comprehensive income attributable to noncontrolling interests	1,233	732	3,391	1,064
Comprehensive income attributable to common stockholders	$ 618,213	$ 144,033	$ 980,670	$ 399,545