Investment Securities - Available for sale sponsored funds with fair values below carrying values (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Fair value
Less than 12 months	$ 61,602	$ 215,186
12 months or longer	131,771
Total temporarily impaired securities	193,373	215,186
Unrealized losses
Less than 12 months	(309)	(1,237)
12 months or longer	(1,698)
Total Unrealized losses on temporarily impaired securities	$ (2,007)	$ (1,237)
Securities in an unrealized loss position | security	46	53
Other-than-temporary impairment of available for sale securities	$ 0	$ 0
Certificates of deposit
Fair value
Less than 12 months		2,998
Total temporarily impaired securities		2,998
Unrealized losses
Less than 12 months		(2)
Total Unrealized losses on temporarily impaired securities		(2)
Corporate bonds
Fair value
Less than 12 months	55,682	192,409
12 months or longer	112,246
Total temporarily impaired securities	167,928	192,409
Unrealized losses
Less than 12 months	(293)	(995)
12 months or longer	(1,210)
Total Unrealized losses on temporarily impaired securities	(1,503)	(995)
U.S. treasury bills
Fair value
Less than 12 months	5,920	19,779
12 months or longer	19,525
Total temporarily impaired securities	25,445	19,779
Unrealized losses
Less than 12 months	(16)	(240)
12 months or longer	(488)
Total Unrealized losses on temporarily impaired securities	$ (504)	$ (240)