Share-Based Plans (Summary of Restricted Stock and Stock Option Activity) (Detail)	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options granted, Number of Shares	124,800
Options exercised, Number of Shares	338,176
Restricted Stock [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)	168,369
Restricted stock, Vested restricted stock awards (in shares)	194,986
Restricted stock, Forfeited (in shares)	18,944
Restricted stock, Outstanding at end of period (in shares)	207,800
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	54.76
Restricted stock, Vested restricted stock awards (in dollars per share) | $ / shares	37.16
Restricted stock, Forfeited (in dollars per share) | $ / shares	35.58
Restricted stock, at end of period, Weighted-average Grant Date Fair Value (in dollars per share) | $ / shares	$ 43.16
Time Based Restricted Stock Unit 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)	2,158
Restricted Stock, Granted (in shares)	8,129
Restricted stock, Vested restricted stock awards (in shares)	10,287
Restricted stock, Forfeited (in shares)	0
Restricted stock, Outstanding at end of period (in shares)	0
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	51.21
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	$ 0
Performance 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)	117,695
Restricted stock, Forfeited (in shares)	9,154
Restricted stock, Outstanding at end of period (in shares)	78,916
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	42.09
Restricted stock, Forfeited (in dollars per share) | $ / shares	43.10
Restricted stock, at end of period, Weighted-average Grant Date Fair Value (in dollars per share) | $ / shares	$ 45.35
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
Options exercised, Number of Shares	399,935
Options forfeited, Number of Shares	0
Options outstanding, at end of period, Number of Shares	673,039
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
Options exercised, Weighted-Average Exercise Price (in dollars per share) | $ / shares	25.42
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	$ 18.75