Stockholders' Equity - Reclassifications from accumulated other comprehensive income (loss) and included in net income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Reclassifications from accumulated other comprehensive income
Reclassifications included in net income:
Pre-tax	$ 928	$ 60
Tax (expense) benefit	(359)	335
Net of tax	569	395
Sponsored funds investment gains | Retained earnings | Reclassifications from accumulated other comprehensive income
Reclassifications included in net income:
Pre-tax	1,295
Tax (expense) benefit	(340)
Net of tax	955
Sponsored funds investment gains | Investment and other income (loss) | Reclassifications from accumulated other comprehensive income
Reclassifications included in net income:
Pre-tax		(124)
Tax (expense) benefit		46
Net of tax		(78)
Losses on available for sale debt securities | Investment and other income (loss) | Reclassifications from accumulated other comprehensive income
Reclassifications included in net income:
Pre-tax	(489)
Tax (expense) benefit	116
Net of tax	(373)
Change in valuation allowance for unrealized gains (losses) on investment securities
Reclassifications included in net income:
Net of tax		355
Change in valuation allowance for unrealized gains (losses) on investment securities | Provision for income taxes | Reclassifications from accumulated other comprehensive income
Reclassifications included in net income:
Tax (expense) benefit		355
Net of tax		355
Postretirement benefits unrealized gains (losses)
Reclassifications included in net income:
Net of tax	94	118
Postretirement benefits unrealized gains (losses) | Underwriting and distribution expense and Compensation and related costs | Reclassifications from accumulated other comprehensive income
Reclassifications included in net income:
Pre-tax	122	184
Tax (expense) benefit	(135)	(66)
Net of tax	$ (13)	$ 118