Fair Value Measurement - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets at fair value	$ 33,600,000	$ 28,800,000
Significant transfers between Level 1 and Level 2	0
Level 3 assets	$ 0
5.75% Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate stated on senior notes	5.75%
Senior notes due date	Oct. 15, 2023