Common Stock and Stock Compensation - Stock Options, Assumptions Used (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assumptions used in the pricing model and the weighted average grant-date fair value of stock options granted
Expected volatility	14.60%	16.60%	17.70%
Expected term (in years)	5 years	5 years	5 years
Interest rate	1.50%	0.90%	0.90%
Dividend yield, percentage	4.90%	4.40%	4.20%
Weighted average grant-date fair value (in dollars per share)	$ 2.20	$ 2.93	$ 3.39
Performance Share Plan [Member]
Assumptions used in the pricing model and the weighted average grant-date fair value of stock options granted
Expected volatility	12.60%	12.00%	16.00%
Expected term (in years)	3 years	3 years	3 years
Interest rate	0.60%	0.40%	0.40%
Dividend yield	$ 2.03	$ 1.96	$ 1.89
Weighted average grant-date fair value (in dollars per share)	$ 37.54	$ 40.50	$ 41.99