Investment Securities - Available for sale sponsored funds with fair values below carrying values (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($)
Fair value
Less than 12 months		$ 215,186
Total fair value		215,186
Unrealized losses
Less than 12 months		(1,237)
Total unrealized losses		(1,237)
Securities in an unrealized loss position | security	44
Other-than-temporary impairment of available for sale securities	$ 0
Certificates of deposit
Fair value
Less than 12 months		2,998
Total fair value		2,998
Unrealized losses
Less than 12 months		(2)
Total unrealized losses		(2)
Corporate bonds
Fair value
Less than 12 months	36,302	192,409
12 months or longer	119,480
Total fair value	155,782	192,409
Unrealized losses
Less than 12 months	(160)	(995)
12 months or longer	(1,209)
Total unrealized losses	$ (1,369)	(995)
U.S. treasury bills
Fair value
Less than 12 months		19,779
Total fair value		19,779
Unrealized losses
Less than 12 months		(240)
Total unrealized losses		$ (240)