Equity (Details Textual) - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2015	Mar. 31, 2011
Class of Stock [Line Items]
Preferred stock, shares authorized	8,000
Authorized amount of share repurchases						$ 5,000.0
Repurchase of common stock, at cost, shares			16,800
Payments for repurchase of common stock	$ 1,365.4	$ 1,000.0	$ 5,000.0
Series A
Class of Stock [Line Items]
Preferred stock, shares authorized	1,750
Series X junior participating
Class of Stock [Line Items]
Preferred stock, shares authorized	1,000
Undesignated
Class of Stock [Line Items]
Preferred stock, shares authorized	5,250
2016 Share Repurchase Program
Class of Stock [Line Items]
Authorized amount of share repurchases				$ 3,000.0	$ 5,000.0
Repurchase of common stock, at cost, shares	3,700	3,300
Payments for repurchase of common stock	$ 1,000.0
2011 Share Repurchase Program
Class of Stock [Line Items]
Common stock shares authorized for repurchase							20,000
Repurchase of common stock, at cost, shares	1,200
Payments for repurchase of common stock	$ 365.4