Stockholders' Equity - Reclassifications from accumulated other comprehensive income (loss) and included in net income (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassifications from accumulated other comprehensive income
Reclassifications included in net income:
Pre-tax	$ (27)	$ 744	$ 1,329	$ 1,090
Tax (expense) benefit	7	(16)	(455)	(51)
Net of tax	(20)	728	874	1,039
Sponsored funds investment gains \| Investment and other income (loss) \| Reclassifications from accumulated other comprehensive income
Reclassifications included in net income:
Pre-tax		698	1,295	952
Tax (expense) benefit		(260)	(340)	(355)
Net of tax		438	955	597
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	(58)		(58)
Tax (expense) benefit	14		14
Net of tax	(44)		(44)
Change in valuation allowance for unrealized gains (losses) on investment securities
Reclassifications included in net income:
Net of tax		260		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		260		355
Net of tax		260		355
Postretirement benefits unrealized gains (losses)
Reclassifications included in net income:
Net of tax	24	30	70	87
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	31	46	92	138
Tax (expense) benefit	(7)	(16)	(129)	(51)
Net of tax	$ 24	$ 30	$ (37)	$ 87

Amount of tax expense (benefit) allocated to other comprehensive income (loss) attributable to parent entity.

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