Quarterly Data (Unaudited) - Quarterly Composite Common Stock Prices and Dividends (Details) - $ / shares	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016
Class of Stock [Line Items]
Dividends	$ 0.328	$ 0.328	$ 0.328	$ 0.328	$ 0.318	$ 0.318	$ 0.318	$ 0.31
Close	39.41	37.67	36.07	35.98	35.49	33.29	36.36	35.86
Maximum [Member]
Class of Stock [Line Items]
Close	42.67	38.78	37.44	37.19	36.36	37.70	36.37	35.95
Minimum [Member]
Class of Stock [Line Items]
Close	$ 37.55	$ 35.37	$ 33.30	$ 34.47	$ 29.82	$ 32.86	$ 33.18	$ 26.26