Fair Value Measurement (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets and liabilities measured at fair value
Total	$ 0	$ 16,744
Total investments at fair value	28,593	40,459
Level 1
Assets and liabilities measured at fair value
Total	0	16,744
Level 2
Assets and liabilities measured at fair value
Total	0	0
Level 3
Assets and liabilities measured at fair value
Total	0	0
Interest in registered investment companies
Assets and liabilities measured at fair value
Deferred compensation investments valued using net asset value as a practical expedient:	14,921	10,140
Fixed income fund
Assets and liabilities measured at fair value
Deferred compensation investments valued using net asset value as a practical expedient:	13,672	13,575
Available for sale investment
Assets and liabilities measured at fair value
Total	0	16,744
Available for sale investment | Level 1
Assets and liabilities measured at fair value
Total	0	16,744
Available for sale investment | Level 2
Assets and liabilities measured at fair value
Total	0	0
Available for sale investment | Level 3
Assets and liabilities measured at fair value
Total	$ 0	$ 0