Supplementary Information Concerning P&C Insurance Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental Information For Property Casualty Insurance Underwriters [Abstract]
Deferred Policy Acquisition Costs	$ 158,080	$ 135,249	$ 119,660
Reserve for Losses and LAE	2,743,973	2,515,145	2,289,727
Unearned Premiums	1,102,735	987,681	887,344
Net Earned Premiums	1,363,223	1,186,420	1,100,345
Net Investment Income	103,113	89,293	79,451
Losses and LAE Expenses Incurred Related to Current Year	817,537	771,955	693,976
Losses and LAE Expenses Incurred Related to Prior Years	76,242	34,310	(28,528)
Amortization of Deferred Policy Acquisition Costs	220,144	184,731	165,045
Other Operating Expenses	261,224	233,230	234,096
Net Written Premiums	$ 1,471,085	$ 1,271,330	$ 1,186,224