Fair Value Measurements - Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Total	$ 65,620	$ 135,204
Liabilities:
Subsidiary unit awards	3,160	2,768
Contingent consideration liability from acquisition	0	0
Total	3,160	2,768
Money market account
Assets:
Money market account	65,620	135,204
Level 1
Assets:
Total	65,620	135,204
Liabilities:
Subsidiary unit awards	0	0
Contingent consideration liability from acquisition	0	0
Total	0	0
Level 1 | Money market account
Assets:
Money market account	65,620	135,204
Level 2
Assets:
Total	0	0
Liabilities:
Subsidiary unit awards	0	0
Contingent consideration liability from acquisition	0	0
Total	0	0
Level 2 | Money market account
Assets:
Money market account	0	0
Level 3
Assets:
Total	0	0
Liabilities:
Subsidiary unit awards	3,160	2,768
Contingent consideration liability from acquisition	0	0
Total	3,160	2,768
Level 3 | Money market account
Assets:
Money market account	$ 0	$ 0