Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Acquired intangible assets
Gross Carrying Amount		$ 111,900
Accumulated Amortization	$ 123,408	110,908
Net Carrying Amount	111,224	992
Gross Carrying Amount	234,633
Net Carrying Amount	111,224
Amortization expense relating to finite-lived intangible assets	15,447	211	$ 211
Exchange memberships
Acquired intangible assets
Indefinite-lived intangibles	5,838
Purchased technology
Acquired intangible assets
Gross Carrying Amount	110,000	110,000
Accumulated Amortization	$ 110,000	$ 110,000
Purchased technology | Minimum
Acquired intangible assets
Useful Lives	1 year 4 months 24 days	1 year 4 months 24 days
Purchased technology | Maximum
Acquired intangible assets
Useful Lives	2 years 6 months	2 years 6 months
ETF issuer relationships
Acquired intangible assets
Gross Carrying Amount	$ 950	$ 950
Accumulated Amortization	559	454
Net Carrying Amount	$ 391	$ 496
Useful Lives	9 years	9 years
ETF buyer relationships
Acquired intangible assets
Gross Carrying Amount	$ 950	$ 950
Accumulated Amortization	559	454
Net Carrying Amount	$ 390	$ 496
Useful Lives	9 years	9 years
Leases
Acquired intangible assets
Gross Carrying Amount	$ 1,800
Accumulated Amortization	397
Net Carrying Amount	$ 1,403
Useful Lives	3 years
FCC licenses
Acquired intangible assets
Gross Carrying Amount	$ 200
Accumulated Amortization	19
Net Carrying Amount	$ 181
Useful Lives	7 years
Technology
Acquired intangible assets
Gross Carrying Amount	$ 60,000
Accumulated Amortization	9,644
Net Carrying Amount	$ 50,356
Technology | Minimum
Acquired intangible assets
Useful Lives	1 year
Technology | Maximum
Acquired intangible assets
Useful Lives	6 years
Customer relationships
Acquired intangible assets
Gross Carrying Amount	$ 49,000
Accumulated Amortization	1,822
Net Carrying Amount	$ 47,178
Customer relationships | Minimum
Acquired intangible assets
Useful Lives	12 years
Customer relationships | Maximum
Acquired intangible assets
Useful Lives	17 years
Favorable leases
Acquired intangible assets
Gross Carrying Amount	$ 5,895
Accumulated Amortization	408
Net Carrying Amount	$ 5,487
Useful Lives	7 years