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Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Additional Information) (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Debt Outstanding [Line Items]          
Interest expense on debt $ 9.0 $ 9.7 $ 27.2 $ 28.2  
3.50% Senior Notes          
Debt Outstanding [Line Items]          
Interest rate on debt 3.50%   3.50%   3.50%
5.375% Senior Notes          
Debt Outstanding [Line Items]          
Interest rate on debt 5.375%   5.375%   5.375%
6.05% Subordinated Notes          
Debt Outstanding [Line Items]          
Interest rate on debt 6.05%   6.05%   6.05%
Fair value of the interest rate swap associated with the notes         $ 0.8
7.0% Junior Subordinated Debentures          
Debt Outstanding [Line Items]          
Interest rate on debt 7.00%   7.00%   7.00%
Interest rate swaps | 6.05% Subordinated Notes          
Debt Outstanding [Line Items]          
Fair value of the interest rate swap associated with the notes         $ 0.8