Quarterly Data (Unaudited) - Quarterly Composite Common Stock Prices and Dividends (Details) (USD $)	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013
Quarterly Financial Data [Abstract]
High price	$ 38.93	$ 36.26	$ 37.73	$ 33.94	$ 30.59	$ 32.42	$ 37.8	$ 34.56
Low price	$ 33.07	$ 30.66	$ 33.11	$ 26.45	$ 26.64	$ 29.42	$ 29.84	$ 29.1
Close	$ 37.08	$ 34.09	$ 36.48	$ 33.56	$ 27.39	$ 29.64	$ 30.88	$ 34.48
Dividends	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.525