Consolidated Statements of Common Stockholders' Equity (Parenthetical) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Consolidated Statements of Common Stockholders' Equity
Dividends paid on common stock (in dollars per share)	$ 0.1575	$ 0.1575	$ 0.1575	$ 0.1575	$ 0.15375	$ 0.15375	$ 0.15375	$ 0.15375	$ 0.630	$ 0.615	$ 0.595