Benefit Plans - Options Outstanding and Options Exercisable by Exercise Price with Weighted-Average Remaining Contractual Life for Options Outstanding and Weighted-Average Exercise Price (Detail)	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Exercise Price 20-25 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Lower Range Limit	$ 20
Exercise Price Upper Range Limit	$ 25
Options Outstanding, Number | shares	481,510
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	5 years 1 month 17 days
Options Outstanding, Weighted-Average Grant Date Value	$ 24.59
Options Exercisable, Number | shares	125,505
Options Exercisable, Weighted-Average Grant Date Value	$ 24.58
Exercise Price 25-30 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Lower Range Limit	25
Exercise Price Upper Range Limit	$ 30
Options Outstanding, Number | shares	57,505
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	3 years 1 month 20 days
Options Outstanding, Weighted-Average Grant Date Value	$ 28.05
Options Exercisable, Number | shares	37,300
Options Exercisable, Weighted-Average Grant Date Value	$ 28.91
Exercise Price 30-35 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Lower Range Limit	30
Exercise Price Upper Range Limit	$ 35
Options Outstanding, Number | shares	654,265
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	4 years 11 months 8 days
Options Outstanding, Weighted-Average Grant Date Value	$ 33.07
Options Exercisable, Number | shares	160,635
Options Exercisable, Weighted-Average Grant Date Value	$ 32.09
Exercise Price 35-40 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Lower Range Limit	35
Exercise Price Upper Range Limit	$ 40
Options Outstanding, Number | shares	8,780
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	5 years 3 months 25 days
Options Outstanding, Weighted-Average Grant Date Value	$ 38.61
Options Exercisable, Number | shares	2,240
Options Exercisable, Weighted-Average Grant Date Value	$ 38.18
Exercise Price 40-45 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Lower Range Limit	40
Exercise Price Upper Range Limit	$ 45
Options Outstanding, Number | shares	6,800
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	6 years 8 months 1 day
Options Outstanding, Weighted-Average Grant Date Value	$ 40.37
Exercise Price 20-45 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Lower Range Limit	20
Exercise Price Upper Range Limit	$ 45
Options Outstanding, Number | shares	1,208,860
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	4 years 11 months 8 days
Options Outstanding, Weighted-Average Grant Date Value	$ 29.52
Options Exercisable, Number | shares	325,680
Options Exercisable, Weighted-Average Grant Date Value	$ 28.65