Fair Value Measurements - Fair Value of Company's Fixed Rate Senior Notes and Junior Subordinated Debentures (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
6.375% Senior Notes [Member] | Estimated Fair Value [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Debt	$ 303,188	$ 271,425
6.375% Senior Notes [Member] | Carrying Amount [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Debt	323,610	322,777
Junior Subordinated Debentures [Member] | Estimated Fair Value [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Debt	141,771	131,691
Junior Subordinated Debentures [Member] | Carrying Amount [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Debt	$ 128,916	$ 129,707