Tax Effects of Temporary Differences that Give Rise to Significant Portions of Deferred Tax Assets and Deferred Tax Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Loss and LAE reserves	$ 114,200	$ 212,100
Minimum tax credit carry forward	137,200	28,900
Compensation accruals	97,400	166,700
Unearned premiums	84,200	139,100
OTTI losses	9,600	19,500
State net operating loss carry forward	18,300	25,200
Other	73,700	173,300
Gross deferred tax assets before valuation allowance	534,600	764,800
Valuation allowance	(18,300)	(25,200)
Gross deferred tax assets	516,300	739,600
Deferred tax liabilities:
Net unrealized gains on investments	227,500	125,800
Deferred acquisition costs	98,700	163,300
Purchase accounting adjustments	13,500	30,200
Other	40,100	65,400
Gross deferred tax liabilities	379,800	384,700
Net deferred tax assets	$ 136,489	$ 354,852