Quarterly Data (Unaudited) - Quarterly Composite Common Stock Prices and Dividends (Details) - USD ($)	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014
Quarterly Financial Data [Abstract]
High price	$ 31.37	$ 34.44	$ 34.98	$ 38.25	$ 38.93	$ 36.26	$ 37.73	$ 33.94
Low price	$ 25.09	$ 28.41	$ 31.28	$ 31.71	$ 33.07	$ 30.66	$ 33.11	$ 26.45
Close	$ 27.77	$ 29.7	$ 31.42	$ 33.61	$ 37.08	$ 34.09	$ 36.48	$ 33.56
Dividends	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.31