Fair Value of Financial Instruments - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
6.05% Subordinated Notes
Debt Disclosure [Line Items]
Stated interest rate	6.05%	6.05%
5.375% Senior Notes
Debt Disclosure [Line Items]
Stated interest rate	5.375%	5.375%
7.0% Junior Subordinated Debentures
Debt Disclosure [Line Items]
Stated interest rate	7.00%	7.00%
Fair Value, Measurements, Recurring
Debt Disclosure [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	$ (1,513)	$ (1,208)	$ (2,422)