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Recent Developments Including Mergers and Acquisitions - Additional Information (Detail) $ in Millions |  |  | 9 Months Ended
--- | --- | --- | ---
Sep. 07, 2017 USD ($) | Jul. 31, 2017 shares | Sep. 30, 2017
First Bank & Trust Company [Member] |  |  |
Business Combination Separately Recognized Transactions [Line Items] |  |  |
Date of announcement regarding acquisition agreement | Sep. 07, 2017 |  |
Total assets acquired | $ 280 |  |
Loans | 258 |  |
Deposits | 244 |  |
Equity capital | $ 36 |  |
Expected date of completion of transaction | 2018-01 |  |
First Bank of Chandler [Member] |  |  |
Business Combination Separately Recognized Transactions [Line Items] |  |  |
Date of announcement regarding acquisition agreement | Sep. 07, 2017 |  |
Total assets acquired | $ 90 |  |
Loans | 82 |  |
Deposits | 79 |  |
Equity capital | $ 11 |  |
Expected date of completion of transaction | 2018-01 |  |
COMMON STOCK [Member] |  |  |
Business Combination Separately Recognized Transactions [Line Items] |  |  |
Stock split description |  |  | On July 31, 2017, the Company completed a two-for-one stock split of the Company’s outstanding shares of common stock.
Stock split conversion ratio |  | 2 |
Stock payable in form of dividend, payable date |  | Jul. 31, 2017 |
Stock payable in form of dividend, record date |  | Jul. 17, 2017 |
Additional shares received by stockholders for each share held | shares |  | 1 |

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