Summary of Significant Accounting Policies - Common Stock Stock Options Excluded as Anti-Dilutive from Determination of Diluted Earnings Per Share (Detail) shares in Thousands, EquityUnit in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 EquityUnit $ / shares $ / EquityUnit shares	Sep. 30, 2014 EquityUnit $ / shares $ / EquityUnit shares	Sep. 30, 2015 EquityUnit $ / shares $ / EquityUnit shares	Sep. 30, 2014 $ / shares shares
Earnings Per Share [Abstract]
Weighted average stock options | shares	103	115	106	115
Weighted average exercise price per share | $ / shares	$ 32.94	$ 32.90	$ 32.98	$ 32.90
Weighted average performance share units | EquityUnit	670	257	631
Weighted average grant date fair value per unit	9.13	20.71	9.13