Stockholders' Equity (Details) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2014 class	Dec. 31, 2013	Dec. 31, 2012
Schedule of Capitalization, Equity [Line Items]
Number of classes of preferred stock		2
Preferred stock, dividend rate	4.00%		4.00%
Stock
Shares reserved for issuance under stock plans, dividend reinvestment program and conversion of Preferred and Preference Stock	37,568,025	37,568,025
Preferred Stock
Schedule of Capitalization, Equity [Line Items]
Preferred stock, dividend rate		4.00%
Dividends		2
Shares issued upon conversion	24.24	24.24
Redemption price per share	50	50
Shares authorized to be issued	600,000	600,000
Shares outstanding	24	24
Stock
Shares outstanding		74
Shares outstanding	24	24
Preference Stock
Schedule of Capitalization, Equity [Line Items]
Dividends		2.12
Shares issued upon conversion	16.53	16.53
Redemption price per share	28	28
Shares authorized to be issued	5,000,000	5,000,000
Shares outstanding	20,237	20,237
Stock
Shares outstanding		21,838
Shares outstanding	20,237	20,237
Common Stock
Schedule of Capitalization, Equity [Line Items]
Shares outstanding	201,027,964	201,027,964	202,082,522	200,884,047
Stock
Shares outstanding		202,082,522	200,884,047	199,751,070
Repurchases of common stock		1,863,262
Issuance of common stock		781,032	1,163,668	1,118,089
Conversions to common stock		27,672	34,807	14,888
Shares outstanding	201,027,964	201,027,964	202,082,522	200,884,047
Treasury Stock
Schedule of Capitalization, Equity [Line Items]
Shares outstanding	122,309,948	122,309,948	121,255,390	122,453,865
Stock
Shares outstanding		121,255,390	122,453,865	123,586,842
Repurchases of common stock		1,863,262
Issuance of common stock		781,032	1,163,668	1,118,089
Conversions to common stock		27,672	34,807	14,888
Shares outstanding	122,309,948	122,309,948	121,255,390	122,453,865