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, 2018 $ / 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	349,990
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	4 years 2 months 1 day
Options Outstanding, Weighted-Average Grant Date Value	$ 24.60
Options Exercisable, Number | shares	109,150
Options Exercisable, Weighted-Average Grant Date Value	$ 24.60
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	33,970
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	2 years 2 months 8 days
Options Outstanding, Weighted-Average Grant Date Value	$ 27.99
Options Exercisable, Number | shares	24,460
Options Exercisable, Weighted-Average Grant Date Value	$ 29.10
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	448,300
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	4 years 3 months 25 days
Options Outstanding, Weighted-Average Grant Date Value	$ 33.31
Options Exercisable, Number | shares	165,675
Options Exercisable, Weighted-Average Grant Date Value	$ 33.03
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	7,900
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	4 years 6 months 29 days
Options Outstanding, Weighted-Average Grant Date Value	$ 38.83
Options Exercisable, Number | shares	3,600
Options Exercisable, Weighted-Average Grant Date Value	$ 38.56
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	5,440
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	5 years 8 months 1 day
Options Outstanding, Weighted-Average Grant Date Value	$ 40.37
Options Exercisable, Number | shares	340
Options Exercisable, 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	845,600
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	4 years 2 months 4 days
Options Outstanding, Weighted-Average Grant Date Value	$ 29.57
Options Exercisable, Number | shares	303,225
Options Exercisable, Weighted-Average Grant Date Value	$ 29.71