Summary of Significant Accounting Policies - Narrative (Details)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) investment_type	Sep. 30, 2018 USD ($)	Dec. 31, 2018	Jan. 01, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of types of investments | investment_type				2
Other-than-temporary impairment loss, debt securities, available-for-sale				$ 0
Payments to acquire equity securities	$ 2,000,000
Advertising expense		$ 5,279,000	$ 5,936,000	59,357,000	$ 29,167,000
Advertising production costs		17,000	$ 7,000	166,000	$ 20,000
Operating lease, right-of-use asset		45,513,000		45,513,000
Operating lease, liability		$ 62,790,000		$ 62,790,000
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset							$ 33,953,000
Operating lease, liability							$ 49,395,000
Software development
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Finite-lived intangible assets, useful life				2 years		1 year