Fair value measurement - Financial Instruments Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets:
Investments at fair value	$ 45,302	$ 55,860
Total investments at fair value	69,017	105,289
Level 1
Assets:
Investments at fair value	45,302	55,860
Level 2
Assets:
Investments at fair value	0	0
Level 3
Assets:
Investments at fair value	0	0
Common collective trust - equities
Assets:
Deferred compensation investments valued using net asset value as a practical expedient:	10,140	36,114
Fixed income fund
Assets:
Deferred compensation investments valued using net asset value as a practical expedient:	13,575	13,315
Deferred compensation investments
Assets:
Investments at fair value	28,558	27,770
Deferred compensation investments | Level 1
Assets:
Investments at fair value	28,558	27,770
Deferred compensation investments | Level 2
Assets:
Investments at fair value	0	0
Deferred compensation investments | Level 3
Assets:
Investments at fair value	0	0
Available for sale investment
Assets:
Investments at fair value	16,744	28,090
Available for sale investment | Level 1
Assets:
Investments at fair value	16,744	28,090
Available for sale investment | Level 2
Assets:
Investments at fair value	0	0
Available for sale investment | Level 3
Assets:
Investments at fair value	$ 0	$ 0