Business Segments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Number of segments | segment	4
Administrative settlements	$ 8,659,000	$ 3,795,000	$ 1,394,000
Administrative settlement, after tax	5,600,000	$ 2,500,000	$ 906,000
Guarantee liability	0
One-time increase in stock-based compensation expense due to Tax Reform	3,380,000
One-time increase in stock-based compensation expense due to Tax Reform, net of tax	2,200,000
Insurance-related assessments
Loss Contingencies [Line Items]
Guarantee liability	9,600,000
Guaranty liability, recoverable amount	$ 1,800,000