CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 80,098	$ 199,148	$ 1,361,112
Other comprehensive income (loss):
Foreign currency translation	12,203	4,082
Unrealized loss on government-sponsored pooled loan investments	(172)	(123)
Other	8,615	(82)
Total other comprehensive income (loss)	20,646	3,877	$ 22,414
Comprehensive income	100,744	203,025
Comprehensive income attributable to noncontrolling interests	1,395	1,021
Comprehensive income attributable to common stockholders	$ 99,349	$ 202,004