Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Equity securities held	$ 25,000,000	$ 25,000,000
Net gain on sale of equity securities	10,000,000
Impairment losses	0	1,000,000	$ 7,000,000
Advertising expenses	$ 18,000,000	$ 18,000,000	$ 18,000,000
Unearned compensation cost, expected to be recognized over period (years)	1 year 6 months