Income Taxes (Details of Income Taxes) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Income before income taxes	$ 150,900,000	$ 245,200,000	$ 192,300,000	$ 385,800,000
Income tax provision	$ 34,400,000	$ 65,400,000	$ 41,400,000	$ 97,200,000
Effective tax rate	22.80%	26.70%	21.50%	25.20%
Unrecognized tax benefits	$ 261,700,000		$ 261,700,000
Unrecognized tax benefits could decrease up to	$ 50,000,000.0		$ 50,000,000.0