Income Taxes - Summary of Effective Tax Rate for Continuing Operations Differed from U.S. Statutory Federal Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Statutory federal tax rate	35.00%	35.00%	35.00%
State taxes, net of federal benefit	(4.00%)	1.10%	4.30%
Valuation allowance on capital loss carryforward and state tax credit carryforward			(2.10%)
Adjustment for final positions on filed returns	(1.10%)	0.20%	(3.50%)
Nondeductible expenses and other, net	1.50%	1.80%	6.10%
Federal tax-exempt interest income			(0.10%)
Federal and state tax credits	(7.70%)	(11.60%)	(14.30%)
Change in federal tax rate	3.20%
Other, net	(0.10%)		2.10%
Total	26.80%	26.50%	27.50%