Goodwill and Intangible Assets, net - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Other expense - goodwill impairment		$ 28,025	$ 0	$ 0
Retirement
Goodwill [Line Items]
Other expense - goodwill impairment	$ 28,000	28,025
Benefit Consultants Group Inc.
Goodwill [Line Items]
Acquired finite-lived intangible assets		14,100
NTA
Goodwill [Line Items]
Acquired finite-lived intangible assets		$ 160,400