Summary of Calculation of Realized Gains and Losses (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Net gains and losses recognized during the period on equity securities	$ (4,374)
Less: Net gains and losses recognized during the period on equity securities sold during the period	554
Unrealized gains and losses recognized during the reporting period on equity securities still held at the reporting date	$ (4,928)