Quarterly report pursuant to Section 13 or 15(d)

Equity Method Investments in Affiliate - Summary of Consolidated Financial Position and Results of Operations of Significant Holding Companies, Accounted for Using Equity Method (Detail)

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Equity Method Investments in Affiliate - Summary of Consolidated Financial Position and Results of Operations of Significant Holding Companies, Accounted for Using Equity Method (Detail) - Strong Upwind Holdings LLC [Member] - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2015
Dec. 31, 2014
Schedule Of Results Related To Equity Accounted Investees [Line Items]    
Current Assets $ 57 $ 62
Total Assets 1,450 1,501
Current Liabilities 13 18
Total Liabilities 61 66
Members' Equity 1,389 1,435
Revenue 77 154
Income from Continuing Operations 21 44
Net Income $ 21 $ 44