Stockholders' Equity (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2015 shares	Dec. 31, 2015 class $ / shares shares	Dec. 31, 2014 shares	Dec. 31, 2013 shares	Dec. 31, 2015 $ / shares shares
Schedule of Capitalization, Equity [Line Items]
Number of classes of preferred stock | class		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					35,377,289
Preferred Stock
Schedule of Capitalization, Equity [Line Items]
Preferred stock, dividend rate		4.00%
Dividends | $ / shares		$ 2
Shares issued upon conversion					24.24
Redemption price per share | $ / shares					$ 50
Shares authorized to be issued					600,000
Shares outstanding	12	24	24		12
Stock
Shares outstanding		24
Shares outstanding	12	12	24
Preference Stock
Schedule of Capitalization, Equity [Line Items]
Dividends | $ / shares		$ 2.12
Shares issued upon conversion					16.53
Redemption price per share | $ / shares					$ 28
Shares authorized to be issued					5,000,000
Shares outstanding	18,660	20,237	20,237		18,660
Stock
Shares outstanding		20,237
Shares outstanding	18,660	18,660	20,237
Common Stock
Schedule of Capitalization, Equity [Line Items]
Shares outstanding	195,521,208	201,027,964	202,082,522	200,884,047	195,521,208
Stock
Shares outstanding		201,027,964	202,082,522	200,884,047
Issuance of common stock		943,686	781,032	1,163,668
Repurchases of common stock		(6,476,796)	(1,863,262)
Conversions to common stock		26,354	27,672	34,807
Shares outstanding	195,521,208	195,521,208	201,027,964	202,082,522
Treasury Stock
Schedule of Capitalization, Equity [Line Items]
Shares outstanding	127,816,704	122,309,948	121,255,390	122,453,865	127,816,704
Stock
Shares outstanding		122,309,948	121,255,390	122,453,865
Issuance of common stock		943,686	781,032	1,163,668
Repurchases of common stock		(6,476,796)	(1,863,262)
Conversions to common stock		26,354	27,672	34,807
Shares outstanding	127,816,704	127,816,704	122,309,948	121,255,390