Income Taxes - Difference of Income Taxes from Statutory Federal Income Tax Rates (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal U.S. income tax rate	35.00%	35.00%	35.00%
State income taxes, net of federal tax benefit	3.70%	4.20%	4.20%
Non-deductible expenses	0.40%	0.50%	0.50%
Non-U.S. income taxed at different rates, net of foreign tax credits	(0.00%)	(0.60%)	0.10%
Federal tax credits	(1.30%)	(0.80%)	(0.60%)
Tax impact of uncertain tax positions	0.20%	0.00%	(0.20%)
Valuation allowance release, net	0.00%	(0.10%)	(0.50%)
Tax effects of TCJA	6.50%	0.00%	0.00%
Other, net	(0.60%)	(0.20%)	(0.10%)
Effective tax rate	43.90%	38.00%	38.40%