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Changes in Option Awards All of Which Related to Common Stock (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-Avg. Per Share Exercise Price, beginning period	$ 21.69
Weighted-Avg. Per Share Exercise Price, ending period	27.73	$ 21.69
Weighted-Avg. Per Share Exercisable Price, ending period	$ 20.48
Outstanding, Beginning Balance	2,664,333	2,653,558	3,154,006
Outstanding, Ending Balance	2,263,126	2,664,333	2,653,558
Exercisable at End of Period	1,387,259	1,602,651	1,544,186
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-Avg. Per Share Exercise Price - Granted	$ 50.16	$ 31.49	$ 23.24
Weighted-Avg. Per Share Exercise Price - Exercised	4.75	4.75	4.10
Weighted-Avg. Per Share Exercise Price - Forfeited	30.77	23.24	17.46
Weighted-Avg. Per Share Exercise Price, ending period	4.75
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-Avg. Per Share Exercise Price - Granted	50.16	46.93	25.34
Weighted-Avg. Per Share Exercise Price - Exercised	31.67	30.77	17.46
Weighted-Avg. Per Share Exercise Price - Forfeited	50.16	$ 46.93	$ 23.24
Weighted-Avg. Per Share Exercise Price, ending period	$ 50.16
Two Thousand Seventeen
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted	358,150
Exercised	(752,603)
Forfeited	(6,754)
Aggregate Intrinsic Value, Exercised at end of Period	$ 12.7
Two Thousand Sixteen
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted		553,370
Exercised		(531,933)
Forfeited		(10,662)
Aggregate Intrinsic Value, Exercised at end of Period		$ 7.5
Two Thousand Fifteen
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted			484,990
Exercised			(978,208)
Forfeited			(7,230)
Aggregate Intrinsic Value, Exercised at end of Period			$ 10.2

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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-Publisher FASB
-Name Accounting Standards Codification
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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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