Total Shareholder Return Percentiles and Number of Shares Issued (Detail) - Performance Units Awards	12 Months Ended
	Dec. 31, 2016 shares
2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total shareholder return percentile for performance period	87.00%
Shares issued	384,000
2011
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total shareholder return percentile for performance period	94.00%
Shares issued	288,750