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Fair value of financial instruments - credit valuation (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Recurring measurements of fair value hierarchy of financial assets and liabilities          
Gain (Loss) on Derivative Instruments, Net, Pretax $ (1.9) $ 9.0 $ (5.8) $ 20.0  
Recurring | Credit valuation adjustments          
Recurring measurements of fair value hierarchy of financial assets and liabilities          
Net increase in fair value     2.4   $ 3.8
Pre-tax gain in other comprehensive income     0.2   0.3
Gain (Loss) on Derivative Instruments, Net, Pretax     $ 2.2   $ 3.5