Share-Based Plans (Summary of Restricted Stock and Stock Option Activity) (Detail)	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options exercisable, Number of Shares	793,630
Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Restricted stock, Outstanding at beginning of period (in shares)	253,361
Restricted stock, Granted (in shares)	164,953
Restricted stock, Vested restricted stock awards (in shares)	155,390	[1]
Restricted stock, Forfeited (in shares)	14,586
Restricted stock, Outstanding at end of period (in shares)	248,338
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Restricted stock, at beginning of period, Weighted-average Grant Date Fair Value (in dollars per share) | $ / shares	$ 32.24
Restricted stock, Granted (in dollars per share) | $ / shares	54.79
Restricted stock, Vested restricted stock awards (in dollars per share) | $ / shares	36.36	[1]
Restricted stock, Forfeited (in dollars per share) | $ / shares	36.44
Restricted stock, at end of period, Weighted-average Grant Date Fair Value (in dollars per share) | $ / shares	$ 43.96
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Restricted stock, Outstanding at beginning of period (in shares)	2,158
Restricted stock, Granted (in shares)	8,129
Restricted stock, Vested restricted stock awards (in shares)	6,900	[1]
Restricted stock, Forfeited (in shares)	0
Restricted stock, Outstanding at end of period (in shares)	3,387
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Restricted stock, at beginning of period, Weighted-average Grant Date Fair Value (in dollars per share) | $ / shares	$ 32.89
Restricted stock, Granted (in dollars per share) | $ / shares	56.07
Restricted stock, Vested restricted stock awards (in dollars per share) | $ / shares	48.82	[1]
Restricted stock, Forfeited (in dollars per share) | $ / shares	0
Restricted stock, at end of period, Weighted-average Grant Date Fair Value (in dollars per share) | $ / shares	$ 56.07
Performance-Based Restricted Stock Awards Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Restricted stock, Outstanding at beginning of period (in shares)	116,184
Restricted stock, Granted (in shares)	89,581
Restricted stock, Vested restricted stock awards (in shares)	87,982	[1]
Restricted stock, Forfeited (in shares)	6,276
Restricted stock, Outstanding at end of period (in shares)	111,507
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Restricted stock, at beginning of period, Weighted-average Grant Date Fair Value (in dollars per share) | $ / shares	$ 33.62
Restricted stock, Granted (in dollars per share) | $ / shares	56.18
Restricted stock, Vested restricted stock awards (in dollars per share) | $ / shares	40.74	[1]
Restricted stock, Forfeited (in dollars per share) | $ / shares	42.72
Restricted stock, at end of period, Weighted-average Grant Date Fair Value (in dollars per share) | $ / shares	$ 45.61
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options outstanding, at beginning of period, Number of Shares	1,072,974
Options granted, Number of Shares	0
Exercised options, Number of Shares	279,344
Options forfeited, Number of Shares	0
Options outstanding, at end of period, Number of Shares	793,630
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Options, at beginning of period, Weighted-average Exercise Price (in dollars per share) | $ / shares	$ 21.24
Options granted, Weighted-Average Exercise Price (in dollars per share) | $ / shares	0
Exercised options, Weighted-Average Exercise Price (in dollars per share) | $ / shares	25.80
Options forfeited, Weighted-Average Exercise Price (in dollars per share) | $ / shares	0
Options, at end of period, Weighted-average Exercise Price (in dollars per share) | $ / shares	$ 19.63

[1] ested for purposes of recording compensation expense.