Fair Value Measurements (Fair Value of Debt) (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Synthetic bonds due 2021 | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[1]	$ 619.4	$ 663.2
Synthetic bonds due 2021 | Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[2]	$ 490.9	$ 428.0
2.00% Senior Notes due 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Senior notes, interest rate, as a percent		2.00%	2.00%
2.00% Senior Notes due 2017 | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[1]	$ 300.0
2.00% Senior Notes due 2017 | Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[2]	$ 300.0
3.45% Senior Notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Senior notes, interest rate, as a percent		3.45%	3.45%
3.45% Senior Notes due 2022 | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[1]	$ 501.5
3.45% Senior Notes due 2022 | Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[2]	$ 500.0
5.00% Notes due 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Senior notes, interest rate, as a percent		5.00%	5.00%
5.00% Notes due 2020 | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[1]	$ 263.7	$ 237.7
5.00% Notes due 2020 | Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[2]	$ 236.3	$ 209.7
3.40% Notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Senior notes, interest rate, as a percent		3.40%	3.40%
3.40% Notes due 2022 | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[1]	$ 194.5	$ 177.6
3.40% Notes due 2022 | Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[2]	$ 177.9	$ 158.0
3.15% Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Senior notes, interest rate, as a percent		3.15%	3.15%
3.15% Notes due 2023 | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[1]	$ 165.1	$ 152.0
3.15% Notes due 2023 | Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[2]	$ 153.3	$ 136.1
3.15% Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Senior notes, interest rate, as a percent		3.15%	3.15%
3.15% Notes due 2023 | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[1]	$ 160.5	$ 142.5
3.15% Notes due 2023 | Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[2]	$ 148.1	$ 131.4
4.00% Notes due 2027
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Senior notes, interest rate, as a percent		4.00%	4.00%
4.00% Notes due 2027 | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[1]	$ 98.6	$ 89.5
4.00% Notes due 2027 | Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[2]	$ 88.9	$ 79.0
4.00% Notes due 2032
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Senior notes, interest rate, as a percent		4.00%	4.00%
4.00% Notes due 2032 | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[1]	$ 135.1	$ 122.9
4.00% Notes due 2032 | Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[2]	$ 114.1	$ 101.2
3.75% Notes due 2033
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Senior notes, interest rate, as a percent		3.75%	3.75%
3.75% Notes due 2033 | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[1]	$ 121.9	$ 103.4
3.75% Notes due 2033 | Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[2]	$ 114.8	$ 101.8

[1] Fair values are based on Level 1 quoted market rates, except for the 4.00% Notes due 2027 which is based on Level 2, quoted market rates on similar liabilities with an appropriate credit spread applied.

[2] Carrying amounts are shown net of unamortized debt discounts and premiums and unamortized debt issuance costs.