Investments (Narrative) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) fund	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) fund	Sep. 30, 2014 USD ($)	Dec. 31, 2014 fund
Schedule of Available-for-sale Securities [Line Items]
Net realized gains/losses transferred from accumulated other comprehensive income	$ 1,100,000	$ 1,800,000	$ 2,300,000	$ 12,600,000
Number of affiliated funds holding seed money | fund	198		198		146
Available-for-sale debt securities maturing in one to five years	$ 1,700,000		$ 1,700,000
Available-for-sale debt securities maturing in five to ten years	5,300,000		5,300,000
Gains and losses on trading securities	$ (17,000,000)	$ (3,100,000)	(18,900,000)	2,000,000
Seed Money [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairment charges on seed money investments			$ 0	$ 0