Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Acquired Intangible Assets [Line Items]
Accumulated Amortization	$ 219,239	$ 148,644
Net Carrying Amount	529,638	83,989
Gross Carrying Amount	748,877	232,633
Net Carrying Amount	529,638	83,989
Exchange memberships
Acquired Indefinite-lived Intangible Assets [Line Items]
Exchange memberships	4,882	5,838
Purchased technology
Acquired Intangible Assets [Line Items]
Gross Carrying Amount	110,000	110,000
Accumulated Amortization	110,000	110,000
Net Carrying Amount	$ 0	$ 0
Purchased technology | Minimum
Acquired Intangible Assets [Line Items]
Useful Lives (Years)	1 year 4 months 24 days	1 year 4 months 24 days
Purchased technology | Maximum
Acquired Intangible Assets [Line Items]
Useful Lives (Years)	2 years 6 months	2 years 6 months
ETF issuer relationships
Acquired Intangible Assets [Line Items]
Gross Carrying Amount	$ 950	$ 950
Accumulated Amortization	770	665
Net Carrying Amount	$ 180	$ 285
Useful Lives (Years)	9 years	9 years
ETF buyer relationships
Acquired Intangible Assets [Line Items]
Gross Carrying Amount	$ 950	$ 950
Accumulated Amortization	770	665
Net Carrying Amount	$ 180	$ 285
Useful Lives (Years)	9 years	9 years
Technology
Acquired Intangible Assets [Line Items]
Gross Carrying Amount	$ 136,000	$ 60,000
Accumulated Amortization	58,203	30,185
Net Carrying Amount	$ 77,797	$ 29,815
Technology | Minimum
Acquired Intangible Assets [Line Items]
Useful Lives (Years)	1 year	1 year
Technology | Maximum
Acquired Intangible Assets [Line Items]
Useful Lives (Years)	6 years	6 years
Customer relationships
Acquired Intangible Assets [Line Items]
Gross Carrying Amount	$ 486,600	$ 49,000
Accumulated Amortization	46,456	5,905
Net Carrying Amount	$ 440,144	$ 43,095
Useful Lives (Years)		12 years
Customer relationships | Minimum
Acquired Intangible Assets [Line Items]
Useful Lives (Years)	10 years
Customer relationships | Maximum
Acquired Intangible Assets [Line Items]
Useful Lives (Years)	12 years
Trade name
Acquired Intangible Assets [Line Items]
Gross Carrying Amount	$ 3,600
Accumulated Amortization	1,000
Net Carrying Amount	$ 2,600
Useful Lives (Years)	3 years
Favorable occupancy leases
Acquired Intangible Assets [Line Items]
Gross Carrying Amount	$ 5,895	$ 5,895
Accumulated Amortization	2,040	1,224
Net Carrying Amount	$ 3,855	$ 4,671
Favorable occupancy leases | Minimum
Acquired Intangible Assets [Line Items]
Useful Lives (Years)	3 years	3 years
Favorable occupancy leases | Maximum
Acquired Intangible Assets [Line Items]
Useful Lives (Years)	15 years	15 years