Stock Repurchases (Details) - USD ($) shares in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Stockholders' Equity Note [Abstract]
Stock repurchase program, authorized amount	$ 1,400.0
Stock repurchase program, remaining authorized amount	$ 396.0
Shares repurchased from employees who surrendered shares to satisfy minimum tax withholding obligations	0.2
Shares repurchased from employees who surrendered shares to satisfy minimum tax withholding obligations, value	$ 21.8	$ 10.8