Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements - Fair Value of Company's Fixed Rate Senior Notes and Junior Subordinated Debentures (Detail)

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Fair Value Measurements - Fair Value of Company's Fixed Rate Senior Notes and Junior Subordinated Debentures (Detail) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Jun. 30, 2014
6.375% Senior Notes      
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]      
Interest rate on Senior Notes 6.375% 6.375% 6.375%
6.375% Senior Notes | Carrying Amount      
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]      
Debt $ 323,888 $ 322,777  
6.375% Senior Notes | Estimated Fair Value      
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]      
Debt 304,013 271,425  
Junior Subordinated Debentures | Carrying Amount      
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]      
Debt 128,640 129,707  
Junior Subordinated Debentures | Estimated Fair Value      
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]      
Debt $ 139,831 $ 131,691