Revenue Recognition - Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue Recognition
Total Revenues	$ 270,154	$ 295,338	$ 295,338	$ 529,564	$ 592,953
Investment management fees
Revenue Recognition
Total Revenues	112,870	130,391	130,391	222,632	264,083
Investment management fees - Funds
Revenue Recognition
Total Revenues	109,007		124,766	214,752	252,429
Investment management fees - Institutional
Revenue Recognition
Total Revenues	3,863		5,625	7,880	11,654
Underwriting and distribution fees
Revenue Recognition
Total Revenues	133,495	137,873	137,873	259,740	275,914
Unaffiliated
Revenue Recognition
Total Revenues	16,900		20,706	33,612	42,337
Unaffiliated Rule 12b-1 service and distribution fees
Revenue Recognition
Total Revenues	16,329		20,051	32,511	41,027
Unaffiliated sales commissions on front-end load mutual fund and variable annuity sales
Revenue Recognition
Total Revenues	488		507	926	977
Unaffiliated other revenues
Revenue Recognition
Total Revenues	83		148	175	333
Wealth Management
Revenue Recognition
Total Revenues	116,595		117,167	226,128	233,577
Wealth Management, Fee-based asset allocation product revenues
Revenue Recognition
Total Revenues	70,220		66,580	135,450	132,097
Wealth Management, Rule 12b-1 service and distribution fees
Revenue Recognition
Total Revenues	16,327		18,109	32,015	36,486
Wealth Management, sales commissions on front-end load mutual fund and variable annuity sales
Revenue Recognition
Total Revenues	12,302		13,823	24,322	28,249
Wealth Management, sales commissions on other products
Revenue Recognition
Total Revenues	8,497		9,065	16,103	17,487
Wealth Management, other revenues
Revenue Recognition
Total Revenues	9,249		9,590	18,238	19,258
Shareholder service fees
Revenue Recognition
Total Revenues	$ 23,789	$ 27,074	$ 27,074	$ 47,192	$ 52,956