Commitments and Contingencies - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($)
Loss Contingencies [Line Items]
Maximum professional liability risks reinsured in excess of retention level	$ 75,000,000
Maximum [Member]
Loss Contingencies [Line Items]
Maximum professional liability aggregate policy limit	78,000,000
Minimum [Member]
Loss Contingencies [Line Items]
Maximum professional liability aggregate retention level	$ 3,000,000