Non-controlling Interests (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
Noncontrolling Interest [Line Items]
Non-redeemable non-controlling interests	$ 62	$ 7,978	$ 118	$ 9,610
Total non-controlling interest in net income	261	17,169	22,911	33,487
Non-redeemable non-controlling interest	1,659		1,659		$ 1,564
Redeemable non-controlling interest	108,524		108,524		601,587
CPH acquisition
Noncontrolling Interest [Line Items]
Non-redeemable non-controlling interest	955		955		807
Redeemable non-controlling interest	5,364		5,364		5,364
Other
Noncontrolling Interest [Line Items]
Non-redeemable non-controlling interests	73	48	171	195
Company-sponsored investment funds
Noncontrolling Interest [Line Items]
Non-redeemable non-controlling interests	(11)	7,930	(53)	9,415
Redeemable non-controlling interests	199	$ 9,191	22,793	$ 23,877
Non-redeemable non-controlling interest	704		704		757
Redeemable non-controlling interest	$ 103,160		$ 103,160		$ 596,223