Stockholders' Equity - Reconciliation of Outstanding Shares (Details) - Common stock - USD ($) shares in Millions, $ in Millions	1 Months Ended			6 Months Ended
	May 31, 2018	May 31, 2017	Feb. 29, 2016	Jun. 30, 2018	Jun. 30, 2017
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period				289.0	287.7
Shares issued				2.2	3.6
Treasury stock acquired				(6.6)	(2.7)
Outstanding shares at end of period				284.6	288.6
Common stock share repurchase disclosures
Share repurchase program, maximum authorized amount (in dollars)	$ 300.0	$ 250.0	$ 400.0