Income Tax - Income Tax Expense Differs from Computed Income Tax at Statutory Rates (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Computed income tax at statutory rates	$ 23,477	$ 32,525	$ 31,996
Benefit of net tax-exempt interest income	(56)	(52)	(284)
Differences in tax rates due to multiple jurisdictions	2,353	32	37
Tax (benefit) expense due to a change in estimate	(334)	258	(201)
Effect of income subject to tax-exemption grant	(16,832)	(24,866)	(23,375)
Unrecognized tax (benefit) expense	(3,828)	373	(11,626)
Effect of disallowed operating losses in foreign entities	0	0	103
Other	0	1	15
Income tax expense (benefit)	$ 4,780	$ 8,271	$ (3,335)