Summary of Significant Accounting Policies - Narrative (Details)	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Dec. 31, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) investment_type	Jun. 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		$ 23,845,000	$ 12,048,000	54,078,000	$ 23,231,000
Advertising production costs		101,000	$ 25,000	149,000	$ 13,000
Operating lease, right-of-use asset		44,830,000		44,830,000
Operating lease, liability		$ 61,353,000		$ 61,353,000
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