Income Taxes - Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended							12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of income tax expense from operations to the federal statutory rate
Income taxes computed at federal statutory rate								$ 20,238	$ 134,961	$ 72,259
State income taxes - net of federal benefit								4,313	13,570	7,253
Federal alternative minimum tax and state and local taxes on pass-through entities								1,076	1,072	1,013
Income taxes computed at the effective federal and state statutory rate for pass-through entities not subject to tax								(41,367)	(84,747)	(75,774)
Increase in valuation allowance								43,175	11,194	1,049
Impact of 2017 Tax Act									78,222
Goodwill impairment								6,158
Other								(783)	638
Income tax expense	$ 9,900	$ 14,262	$ 6,865	$ 127,198	$ 8,151	$ 14,081	$ 5,480	30,790	154,910	$ 5,800
Tax Receivable Agreement liability adjustment								300	$ (38,800)
State
Reconciliation of income tax expense from operations to the federal statutory rate
Impact of other state tax rate changes								$ (2,020)