Common Stock and Stock Compensation - Stock Options, Assumptions Used (Details) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assumptions used in the pricing model and the weighted average grant-date fair value of stock options granted
Expected volatility		14.60%	16.60%
Expected term (in years)		5 years	5 years
Interest rate		1.50%	0.90%
Dividend yield, percentage		4.90%	4.40%
Weighted average grant-date fair value (in dollars per share)		$ 2.20	$ 2.93
Performance Shares [Member]
Assumptions used in the pricing model and the weighted average grant-date fair value of stock options granted
Expected volatility	12.90%	12.60%	12.00%
Expected term (in years)	3 years	3 years	3 years
Interest rate	1.00%	0.60%	0.40%
Dividend yield		$ 2.03	$ 1.96
Weighted average grant-date fair value (in dollars per share)	$ 46.38	$ 37.54	$ 40.50