Summary of Significant Accounting Policies (Details) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended	15 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) segment	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) segment	Mar. 31, 2016 USD ($) item	Oct. 07, 2013
Significant Accounting Policies [Line Items]
Number of reportable segments | segment		1		2
Number of brokerages having assets and liabilities sold | item					21
Increase in cash flows provided by operating activities		$ 49,040	$ 59,952
Decrease in cash flows used in financing activities		36,577	69,027
Excess tax benefit	$ 50	201	$ 2,411
Debt issuance costs, net				$ 1,527
Early adoption of accounting pronouncement
Significant Accounting Policies [Line Items]
Cumulative effect adjustment to retained earnings		44
Increase in cash flows provided by operating activities		2,411
Decrease in cash flows used in financing activities		2,411
Excess tax benefit		$ 201
Deferred tax assets, net					$ 3,332
RMCO, LLC
Significant Accounting Policies [Line Items]
Minority economic interest in RMCO (as a percent)		58.42%				39.56%