Portion of Net Unrealized Losses Recognized to Equity Securities (Detail) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Net Unrealized Losses on Equity Securities	$ (33,795)	$ 2,310	$ 1,329	$ (3,181)	$ (33,337)	$ 0	$ 0
Equity Securities
Gain (Loss) on Investments [Line Items]
Total Net Realized and Unrealized Losses recognized					(32,585)
Less: Net Realized Gains on Investments Sold					752
Net Unrealized Losses on Equity Securities					$ (33,337)