Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Revenue Recognition (Detail)

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Summary of Significant Accounting Policies - Revenue Recognition (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Revenue Recognition [Line Items]          
Percent increase (decrease) in operating results due to change in contract estimates 10.20% (9.20%)   (6.70%)  
Change in contract estimates $ 24.6        
Change orders and/or claims $ 118.9   $ 118.9   $ 137.8
Maximum          
Revenue Recognition [Line Items]          
Percent aggregate change in contract estimates impact on operating results is less than this percentage     5.00%    
Alaska Power Plant Construction Project          
Revenue Recognition [Line Items]          
Losses on contracts   $ 30.5   $ 51.8