Description of Business and Significant Accounting Policies - Summary of Correction of Error for Prior Period Presented and to Conform to Current Year Presentation (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Common stock	$ 1,047	$ 957
Additional paid-in capital	$ 2,570,072	2,553,039
Previously Presented
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Common stock		4,784
Additional paid-in capital		2,549,212
Adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Common stock		(3,827)
Additional paid-in capital		$ 3,827