Fair Value Measurements (Debt Instruments) (Details) $ in Millions	Dec. 31, 2017 USD ($)
$750.0 million fixed rate notes due September 2022, stated rate of 3.00%
Debt Instrument, Fair Value Disclosure	$ 764.9
Seven Hundred Fifty Million Fixed Rate Notes Due March Two Thousand Twenty Five, Interest Equal To Three Point Zero Percent [Member] [Member]
Debt Instrument, Fair Value Disclosure	758.2
Seven Hundred Fifty Million Fixed Rate Notes Due September Two Thousand Forty Three, Interest Equal To Five Point Three Percent [Member]
Debt Instrument, Fair Value Disclosure	$ 960.0