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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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