Stockholders' Equity (Deficit) (Details)	12 Months Ended
	Dec. 31, 2017 class $ / shares shares	Dec. 31, 2016 shares	Dec. 31, 2015 shares	Dec. 31, 2017 $ / 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				33,691,570
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	12		12
Stock
Shares outstanding	12
Shares outstanding	12	12
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	17,832	17,832		16,301
Stock
Shares outstanding	17,832
Shares outstanding	16,301	17,832
Common Stock
Schedule of Capitalization, Equity [Line Items]
Shares outstanding	185,668,718	195,521,208	201,027,964	186,603,738
Stock
Shares outstanding	185,668,718	195,521,208	201,027,964
Repurchases of common stock		(10,633,235)	(6,476,796)
Issuance of common stock	881,480	767,060	943,686
Conversions to common stock	53,540	13,685	26,354
Shares outstanding	186,603,738	185,668,718	195,521,208
Treasury Stock
Schedule of Capitalization, Equity [Line Items]
Shares outstanding	137,669,194	127,816,704	122,309,948	136,734,174
Stock
Shares outstanding	137,669,194	127,816,704	122,309,948
Repurchases of common stock		(10,633,235)	(6,476,796)
Issuance of common stock	881,480	767,060	943,686
Conversions to common stock	53,540	13,685	26,354
Shares outstanding	136,734,174	137,669,194	127,816,704