Stockholders' Equity - Reconciliation of Outstanding Shares (Details) - USD ($) shares in Millions, $ in Millions		1 Months Ended					12 Months Ended
	Jun. 30, 2015	May 31, 2017	Feb. 29, 2016	Oct. 31, 2015	Feb. 28, 2015	Feb. 28, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common stock
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period							287.7	291.4	293.9
Shares issued							4.9	3.0	3.5
Treasury stock acquired							(3.6)	(6.7)	(6.0)
Outstanding shares at end of period							289.0	287.7	291.4
Common stock share repurchase disclosures
Share repurchase program, maximum authorized amount (in dollars)		$ 250.0	$ 400.0	$ 150.0	$ 150.0	$ 200.0
Series A | Preferred stock
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period									3.0
Preferred stock redemption	(3.0)								(3.0)
Series B | Preferred stock
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period									10.0
Preferred stock redemption	(10.0)								(10.0)