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Activity of Awarded Restricted Stock Units (Detail)
12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
shares
Restricted Stock Units (RSUs)  
Number of shares  
Vested (in shares) | shares (35,854)
Forfeited (in shares) | shares (9,137)
Weighted-Average Grant Date Fair Value  
Vested (in dollars per share) | $ / shares $ 23.15
Forfeited (in dollars per share) | $ / shares $ 51.71
Performance Restricted Stock Units [Domain]  
Number of shares  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | shares 98,526
Forfeited (in shares) | shares 0
Weighted-Average Grant Date Fair Value  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares $ 57.81
Forfeited (in dollars per share) | $ / shares $ 0.00
Share-based Compensation Arrangements by Share-based Payment Award, Options, Cancelled | $ $ 0
Share-based Compensation Arrangement by Share-by Payment Awards, Options, Weighted Average Price of Shares Canceled | $ / shares $ 0.00
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | shares 98,526
Minimum | Performance Restricted Stock Units [Domain]  
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]  
Share-based Compensation Arrangement by Share-based Payment Award, Shares, Expected to Vest, Percentage 0.00%
Maximum | Performance Restricted Stock Units [Domain]  
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]  
Share-based Compensation Arrangement by Share-based Payment Award, Shares, Expected to Vest, Percentage 200.00%
Successor [Member] | Restricted Stock Units (RSUs)  
Number of shares  
Balance at beginning of year (in shares) | shares 107,563
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | shares 197,418
Weighted-Average Grant Date Fair Value  
Balance at beginning of year (in dollars per share) | $ / shares $ 23.15
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares 48.41
Balance at end of year (in dollars per share) | $ / shares $ 41.32
Successor [Member] | Performance Restricted Stock Units [Domain]  
Number of shares  
Balance at beginning of year (in shares) | shares 0
Balance at end of year (in shares) | shares 98,526
Weighted-Average Grant Date Fair Value  
Balance at beginning of year (in dollars per share) | $ / shares $ 0.00
Balance at end of year (in dollars per share) | $ / shares $ 57.81
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | shares 98,526