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Retirement Plans - Changes in Fair Value of Level Three Assets (Detail) - Fair Value, Inputs, Level 3 - Private Equity - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets at beginning of year $ 47.3 $ 43.4
Unrealized gains - net 11.9 13.6
Purchases, sales and settlements (14.1) (9.7)
Fair value of plan assets at end of year $ 45.1 $ 47.3