Fair Value Measurements - Fair Value of Company's Fixed Rate Senior Notes and Junior Subordinated Debentures (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
6.375% Senior Notes [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Interest rate on Senior notes	6.375%	6.375%
6.375% Senior Notes [Member] | Carrying Amount [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Debt	$ 323,332	$ 322,777
6.375% Senior Notes [Member] | Estimated Fair Value [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Debt	295,350	271,425
Junior Subordinated Debentures [Member] | Carrying Amount [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Debt	129,186	129,707
Junior Subordinated Debentures [Member] | Estimated Fair Value [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Debt	$ 140,337	$ 131,691