Quarterly Data (Unaudited) - Quarterly Composite Common Stock Prices and Dividends (Details) - USD ($)	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015
Quarterly Financial Data [Abstract]
High price	$ 36.36	$ 37.70	$ 36.37	$ 35.95	$ 31.37	$ 34.44	$ 34.98	$ 38.25
Low price	$ 29.82	$ 32.86	$ 33.18	$ 26.26	$ 25.09	$ 28.41	$ 31.28	$ 31.71
Close	$ 35.49	$ 33.29	$ 36.36	$ 35.86	$ 27.77	$ 29.70	$ 31.42	$ 33.61
Dividends	$ 0.318	$ 0.318	$ 0.318	$ 0.310	$ 0.310	$ 0.310	$ 0.310	$ 0.310