Investments - KCG - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Nov. 30, 2016	Dec. 31, 2015	Nov. 30, 2015	Dec. 31, 2014	Nov. 30, 2014
Schedule of Equity Method Investments [Line Items]
Securities borrowed		$ 7,743,562		$ 6,975,136
Securities loaned		$ 2,819,132		2,979,300
KCG Holdings, Inc [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage		24.00%
Changes in fair value of investment, gains		$ 19,600		49,100
Changes in fair value of investment, losses						$ (14,700)
Net earnings from equity method investment			$ 249,100		$ 61,100
Advisory fees		2,900
Securities borrowed		9,200		6,300
Securities loaned		$ 9,200		$ 16,500
Subsequent event [Member] | KCG Holdings, Inc [Member]
Schedule of Equity Method Investments [Line Items]
Net earnings from equity method investment	$ 255,700