Stock Compensation Plans - SSAR's and Stock Option Activity (Detail) - SSAR's and Stock Options [Member] - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Balance at December 31, 2013, shares subject to SSAR's/Options (in shares)	19
Exercised, shares subject to SSAR's/Options (in shares)	(15)
Cancelled, shares subject to SSAR's/Options (in shares)	0
Balance at December 31, 2014, shares subject to SSAR's/Options (in shares)	4	19
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Balance at December 31, 2013, exercise price	$ 59.14
Exercised, exercise price	57.66
Cancelled, exercise price	0.00
Balance at December 31, 2014, exercise price	$ 64.25	$ 59.14
SSARs/ Options Exercisable	4	18