Consolidated Company-Sponsored Investment Funds (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Derivative [Line Items]
Gain (loss) on derivative instruments	$ (419)	$ (5,387)	$ 1,948	$ (14,879)
Cash collateral payable to trade counterparties	300		300		$ 500
Cash collateral received	3,000		3,000		8,800
Net assets of investment products that are non-consolidated VIEs	47,600,000		47,600,000
Maximum risk of loss in investment in VIEs	6,200		6,200
VIEs
Derivative [Line Items]
Futures, forwards and swaps held	400		400		8,400
Gain (loss) on derivative instruments	15,800	$ 7,800	900	$ 15,000
Cash collateral payable to trade counterparties	240		240		194
Cash collateral received	1,100		1,100		2,900
VOEs
Derivative [Line Items]
Futures, forwards and swaps held	300		300		300
Gain (loss) on derivative instruments	1,800		1,300
Cash collateral payable to trade counterparties	136		136		37
Cash collateral received	600		600		$ 200
Seed capital
Derivative [Line Items]
Amount of investment deconsolidated	$ 100,000		$ 100,000