Fair value measurement - Financial Instruments Measured at Fair Value (Detail) $ in Thousands	Dec. 31, 2016 USD ($)
Assets:
Investments at fair value	$ 30,319
Level 1
Assets:
Investments at fair value	30,319
Level 2
Assets:
Investments at fair value	0
Level 3
Assets:
Investments at fair value	0
Interest in registered investment companies
Assets:
Investments at fair value	13,575
Interest in registered investment companies | Level 1
Assets:
Investments at fair value	13,575
Interest in registered investment companies | Level 2
Assets:
Investments at fair value	0
Interest in registered investment companies | Level 3
Assets:
Investments at fair value	0
Available for sale investment
Assets:
Investments at fair value	16,744
Available for sale investment | Level 1
Assets:
Investments at fair value	16,744
Available for sale investment | Level 2
Assets:
Investments at fair value	0
Available for sale investment | Level 3
Assets:
Investments at fair value	$ 0