ITG Acquisition - Pro Forma Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Business Acquisition [Line Items]
Revenue	$ 385,364			$ 295,123			$ 1,126,812	$ 1,438,303
Net income (loss)	(5,184)	$ (55,485)	$ (13,619)	15,610	$ 46,622	$ 410,022	(74,288)	472,255
Net income (loss) available for common stockholders	(4,312)			8,612			(40,876)	208,573
Pro Forma
Business Acquisition [Line Items]
Revenue	385,365			295,124			1,202,071	1,698,264
Net income (loss)	1,482			3,097			(69,408)	383,941
Net income (loss) available for common stockholders	$ (681)			$ 2,801			$ (38,191)	$ 169,569