Acquisition of KCG Holdings, Inc. - Estimated fair values of the assets acquired and liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Jul. 20, 2017	Dec. 31, 2016
Acquisition of KCG Holdings, Inc.
Goodwill	$ 844,883			$ 715,379
KCG
Acquisition of KCG Holdings, Inc.
Cash and equivalents	595,669
Securities borrowed	1,406,444
Securities purchased under agreements to resell	16,894
Receivables from broker dealers and clearing organizations	552,820
Financial instruments owned, at fair value	2,095,339
Property, equipment and capitalized software (net)	114,367
Intangibles	174,995
Deferred taxes	23,908
Other assets	323,184
Total Assets	5,303,620
Securities loaned	166,189
Securities sold under agreements to repurchase	841,606
Payables to broker dealers and clearing organizations	536,653
Financial instruments sold, not yet purchased, at fair value	1,756,647
Accounts payable and accrued expenses and other liabilities	254,528
Debt	480,987
Total Liabilities	4,036,610
Total identified assets acquired, net of assumed liabilities	1,267,010
Goodwill	128,286		$ 128,286
Total Purchase Price	1,395,296
As previously reported | KCG
Acquisition of KCG Holdings, Inc.
Cash and equivalents		$ 592,993
Cash and securities segregated under federal regs		3,000
Securities borrowed		1,406,444
Securities purchased under agreements to resell		16,894
Receivables from broker dealers and clearing organizations		553,031
Financial instruments owned, at fair value		2,095,339
Property, equipment and capitalized software (net)		112,204
Intangibles		156,300
Deferred taxes		22,928
Other assets		331,820
Total Assets		5,290,953
Securities loaned		166,189
Securities sold under agreements to repurchase		841,606
Payables to broker dealers and clearing organizations		536,653
Payables to customers		17,583
Financial instruments sold, not yet purchased, at fair value		1,756,647
Accounts payable and accrued expenses and other liabilities		239,004
Debt		480,987
Total Liabilities		4,038,669
Total identified assets acquired, net of assumed liabilities		1,252,284
Goodwill		143,012
Total Purchase Price		$ 1,395,296
Changes | KCG
Acquisition of KCG Holdings, Inc.
Cash and equivalents	2,676
Cash and securities segregated under federal regs	(3,000)
Receivables from broker dealers and clearing organizations	(211)
Property, equipment and capitalized software (net)	2,163
Intangibles	18,695
Deferred taxes	980
Other assets	(8,636)
Total Assets	12,667
Payables to customers	(17,583)
Accounts payable and accrued expenses and other liabilities	15,524
Total Liabilities	(2,059)
Total identified assets acquired, net of assumed liabilities	14,726
Goodwill	$ (14,726)