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Income Per Share and Stockholders' Equity - Additional Information (Detail) - USD ($) | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Feb. 28, 2019 | Jan. 31, 2019 | Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018 | Oct. 27, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |  |  |  |  |
Shares repurchased during the period, shares |  |  |  |  | 3,980,656 |
Shares repurchased during the period, value |  |  | $ 45,088,000 |  | $ 161,000,000 |
Payments for repurchase of additional shares |  |  | $ 45,088,000 |  |  |
Class A Common Stock [Member] |  |  |  |  |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |  |  |  |  |
Authorized share repurchase term | 3 years |  | 3 years |  |  |
Shares repurchased during the period, value |  |  | $ 11,000 |  |  |
Average price per share |  | $ 39.38 |  |  | $ 40.43 |
Payments for repurchase of additional shares |  | $ 45,100,000 |  |  |  |
Shares repurchased during the period |  | 1,144,449 |  |  |  |
Class A Common Stock [Member] | Maximum [Member] |  |  |  |  |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |  |  |  |  |
Authorized share repurchase amount | $ 300,000,000 |  |  |  |  | $ 250,000,000
Contingently Issuable Shares [Member] |  |  |  |  |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |  |  |  |  |  |
Shares excluded in computation of diluted income per share |  |  | 547,676 | 69,346 |  |

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