Fair Value Measurements - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
G5 [Member]
Fair Value Measurements, Nonrecurring [Line Items]
Held-to-maturity Securities	$ 10,995
G5 [Member] | Equity Method Investments [Member] | Level III [Member]
Fair Value Measurements, Nonrecurring [Line Items]
Assets, Fair Value Disclosure, Nonrecurring		$ 11,555
Atalanta Sosnoff [Member] | Equity Method Investments [Member] | Level III [Member]
Fair Value Measurements, Nonrecurring [Line Items]
Assets, Fair Value Disclosure, Nonrecurring			$ 14,730
Institutional Asset Management [Member] | Level III [Member]
Fair Value Measurements, Nonrecurring [Line Items]
Assets, Fair Value Disclosure, Nonrecurring		$ 14,401