Reinsurance - Effect of Reinsurance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Direct premiums	$ 2,499,584	$ 2,341,088	$ 2,246,595
Reinsurance premiums assumed from other companies	286,165	227,053	194,910
Reinsurance premiums ceded to other companies	(557,556)	(500,939)	(444,857)
Net premiums	$ 2,228,193	$ 2,067,202	$ 1,996,648