Consolidated Statements of Changes in Stockholders' Equity (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016
Stockholders' Equity Attributable to Parent	$ 10,226.0	$ 10,216.2	$ 9,977.7
Preferred Stock
Stockholders' Equity Attributable to Parent		882.0		$ 882.0
Common Stock
Stockholders' Equity Attributable to Parent	408.6	408.6		408.6	$ 408.6
Series C Preferred Stock | Preferred Stock
Stockholders' Equity Attributable to Parent	388.5		388.5	388.5
Series D Preferred Stock | Preferred Stock
Stockholders' Equity Attributable to Parent	$ 493.5	$ 493.5	$ 493.5	$ 493.5