Fair Value of Financial Instruments - Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Additional Information) (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Stated interest rate on debt	3.50%
5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Stated interest rate on debt	5.375%	5.375%
6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Stated interest rate on debt	6.05%	6.05%
Fair value of the interest rate swap associated with the notes	$ 3.8	$ 4.6
7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Stated interest rate on debt	7.00%	7.00%