Fair Value Measurements Fair Value of Debt Instruments (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Apr. 30, 2015
$750.0 Million Fixed Rate Notes Due March 2043, Interest Equal To 5.3% [Member]
Debt Instrument, Face Amount	$ 750.0
Debt Instrument, Maturity Date	Sep. 01, 2043
Debt Instrument, Interest Rate, Stated Percentage	5.30%
$750.0 Million Fixed Rate Notes Due March 2025, Interest Equal To 3.00%
Debt Instrument, Face Amount	$ 750.0
Debt Instrument, Maturity Date	Mar. 01, 2025
Debt Instrument, Interest Rate, Stated Percentage	3.00%
$750 Million Fixed Rate Notes Due March 2022, Interest Equal To 3.0% [Member]
Debt Instrument, Face Amount	$ 750.0
Debt Instrument, Maturity Date	Sep. 01, 2022
Debt Instrument, Interest Rate, Stated Percentage	3.00%
$612.5 million fixed rate notes due March 2018, interest equal to 4.40% [Member]
Debt Instrument, Face Amount	$ 612.5	$ 612.5
Debt Instrument, Maturity Date	Mar. 01, 2018
Debt Instrument, Interest Rate, Stated Percentage	4.40%
Fair Value, Inputs, Level 2 [Member] | $750.0 Million Fixed Rate Notes Due March 2043, Interest Equal To 5.3% [Member]
Debt Instrument, Fair Value Disclosure	$ 851.7
Fair Value, Inputs, Level 2 [Member] | $750.0 Million Fixed Rate Notes Due March 2025, Interest Equal To 3.00%
Debt Instrument, Fair Value Disclosure	733.8
Fair Value, Inputs, Level 2 [Member] | $750 Million Fixed Rate Notes Due March 2022, Interest Equal To 3.0% [Member]
Debt Instrument, Fair Value Disclosure	$ 753.9