Income Taxes (Reconciliation of Federal Statutory Rate) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Difference between the effective tax rate implicit in the consolidated financial statements and the statutory federal income tax rate
Federal income tax provision at statutory rate (as a percent)	35.00%	35.00%	35.00%
State franchise taxes, net of federal tax effect (as a percent)	5.90%	6.10%	6.30%
Tax Cuts and Jobs Act (as a percent)	4.50%	0.00%	0.00%
Tax credits, net of amortization (as a percent)	(15.10%)	(18.30%)	(8.70%)
Other, net (as a percent)	0.90%	1.80%	0.90%
Effective income tax rate (as a percent)	31.20%	24.60%	33.50%