Changes in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	$ 16,542	$ (22,475)	$ (21,231)	$ 8,983
Other comprehensive income (loss) before reclassification, before tax	10,967	(2,399)	56,291	(27,482)
Amounts reclassified from accumulated other comprehensive (income) loss, before tax	(946)	835	(2,834)	1,660
Other comprehensive income (loss), before tax	10,021	(1,564)	53,457	(25,822)
Income tax (expense) benefit	(1,249)	210	(6,912)	3,038
Cumulative effect adjustment, net of tax				(10,028)
Ending balance, net of tax	25,314	(23,829)	25,314	(23,829)
Unrealized Gains and Losses on Available for Sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	17,745	(21,926)	(19,897)	8,272
Other comprehensive income (loss) before reclassification, before tax	10,767	(1,945)	55,960	(25,928)
Amounts reclassified from accumulated other comprehensive (income) loss, before tax	(946)	835	(2,834)	1,660
Other comprehensive income (loss), before tax	9,821	(1,110)	53,126	(24,268)
Income tax (expense) benefit	(1,249)	210	(6,912)	3,038
Cumulative effect adjustment, net of tax				(9,868)
Ending balance, net of tax	26,317	(22,826)	26,317	(22,826)
Foreign Currency Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	(1,203)	(549)	(1,334)	711
Other comprehensive income (loss) before reclassification, before tax	200	(454)	331	(1,554)
Amounts reclassified from accumulated other comprehensive (income) loss, before tax	0	0	0	0
Other comprehensive income (loss), before tax	200	(454)	331	(1,554)
Income tax (expense) benefit	0	0	0	0
Cumulative effect adjustment, net of tax				(160)
Ending balance, net of tax	$ (1,003)	$ (1,003)	$ (1,003)	$ (1,003)