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Preferred and Common Stockholder's Equity - Schedule of Stock Repurchases (Detail) - USD ($) shares in Millions, $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2018 | Jun. 30, 2019
Stockholders Equity [Line Items] |  |  |
Shares purchased and retired |  |  | 1.6
Common stock and additional paid-in-capital reductions | $ 16.9 | $ 16.9 |
Charges to retained earnings | $ 33.1 | $ 33.1 |
Common Stock [Member] |  |  |
Stockholders Equity [Line Items] |  |  |
Shares purchased and retired | 1.6 | 1.6 |

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