Stockholders' Equity - Reconciliation of Outstanding Shares (Details) - Common stock - USD ($) shares in Millions, $ in Millions	1 Months Ended			6 Months Ended
	May 31, 2017	Feb. 29, 2016	Oct. 31, 2015	Jun. 30, 2017	Jun. 30, 2016
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period				287.7	291.4
Shares issued				3.6	1.9
Treasury stock acquired				(2.7)	(5.3)
Outstanding shares at end of period				288.6	288.0
Common stock share repurchase disclosures
Share repurchase program, maximum authorized amount (in dollars)	$ 250.0	$ 400.0	$ 150.0