Share-Based Compensation Plans - Changes in Nonvested Stock Options (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share
Nonvested stock options outstanding, at beginning of the year (in shares)	1,259,160
Granted (in shares)	468,381
Vested (in shares)	(471,089)
Forfeited or Cancelled (in shares)	(9,947)
Nonvested stock options outstanding, at end of the year (in shares)	1,246,505	1,259,160
Weighted-average grant-date fair value per share
Nonvested stock options weighted average grant date fair value at beginning of the year (in dollars per share)	$ 15.89
Granted (in dollars per share)	19.18	$ 14.84	$ 18.72
Vested (in dollars per share)	15.55
Forfeited or Cancelled (in dollars per share)	16.17
Nonvested stock options weighted average grant date fair value at end of the year (in dollars per share)	$ 17.25	$ 15.89