Quarterly report pursuant to Section 13 or 15(d)

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

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Equity Method Investment in Affiliate - Summary of Consolidated Financial Position and Results of Operations of Holding Companies, Accounted for Using Equity Method (Detail) - Strong Upwind Holdings LLC [Member] - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Schedule Of Results Related To Equity Accounted Investees [Line Items]    
Current Assets $ 56 $ 62
Total Assets 1,475 1,501
Current Liabilities 9 18
Total Liabilities 57 66
Members' Equity 1,418 1,435
Revenue 40 154
Income from Continuing Operations 14 44
Net Income $ 14 $ 44