Consolidated Statements of Common Stockholders' Equity (Parenthetical) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Consolidated Statements of Common Stockholders' Equity
Dividends paid on common stock (in dollars per share)	$ 0.1600	$ 0.1600	$ 0.1600	$ 0.1600	$ 0.1575	$ 0.1575	$ 0.1575	$ 0.1575	$ 0.640	$ 0.630	$ 0.615