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Stock Option Plans - Additional information (Detail) - USD ($) $ / shares in Units, $ in Millions				12 Months Ended
	Mar. 17, 2017	Mar. 16, 2017	Mar. 11, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Option Plans [Line Items]
Maximum number of shares available				476,350	479,167	394,975
Compensation expense related to stock option grants				$ 17.3	$ 14.7	$ 11.2
Total intrinsic value of options exercised				32.7	$ 19.3	$ 27.0
Total unrecognized compensation cost related to nonvested options				$ 42.8
Weighted average period, years				4 years
Long Term Incentive Plan [Member]
Stock Option Plans [Line Items]
Maximum number of shares available				4,000,000
Shares available for grant				16,200,000
2014 Long Term Incentive Plan [Member]
Stock Option Plans [Line Items]
Number of years options expire from date of grant				7 years	7 years	7 years
Officer and Key Employees [Member] | 2014 Long Term Incentive Plan [Member]
Stock Option Plans [Line Items]
Shares available for grant	2,576,000	1,650,400	1,941,000
Stock options granted, exercise percentage, on the third anniversary date of the grant	34.00%	34.00%	34.00%
Stock options granted, exercise percentage, on the fourth anniversary date of the grant	33.00%	33.00%	33.00%
Stock options granted, exercise percentage, on the fifth anniversary date of the grant	33.00%	33.00%	33.00%
Maximum [Member] | Long Term Incentive Plan [Member]
Stock Option Plans [Line Items]
Maximum period for the exercise of stock options, years				7 years
Minimum [Member] | Long Term Incentive Plan [Member]
Stock Option Plans [Line Items]
Minimum exercise price of stock options, percent of fair market value of a share of common stock on the date of grant				100.00%
Minimum [Member] | 2014 Long Term Incentive Plan [Member]
Stock Option Plans [Line Items]
Period of service from grant date stock options awarded not subject to forfeiture				2 years	2 years	2 years
Minimum [Member] | Executive Officer [Member] | 2014 Long Term Incentive Plan [Member]
Stock Option Plans [Line Items]
Minimum age of employee with not subject to award forfeiture on condition compliance				55 years	55 years	55 years
Black-Scholes Option Pricing Model [Member]
Stock Option Plans [Line Items]
Weighted average fair value per option for all options				$ 11.42	$ 8.45	$ 9.25

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-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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