Stock-Based Compensation Stock-Based Compensation (Activity Pertaining to Nonemployee Director Stock Award Plan) (Details) - Directors Plan - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, beginning balance	108,500	129,500	150,500
Weighted average price, beginning balance	$ 52.56	$ 54.15	$ 54.18
Number of shares, granted	0	0	0
Weighted average price, granted	$ 0.00	$ 0.00	$ 0.00
Number of shares, exercised	0	0	0
Weighted average price, exercised	$ 0.00	$ 0.00	$ 0.00
Number of shares, forfeited	(21,000)	(21,000)	(21,000)
Weighted average price, forfeited	$ 57.63	$ 62.40	$ 54.35
Number of shares, ending balance	87,500	108,500	129,500
Weighted average price, ending balance	$ 51.34	$ 52.56	$ 54.15