Fair Value Measurements (Fair Value of Debt) (Details) - USD ($) $ in Millions		Jun. 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]	$ 586.7	$ 663.2
Synthetic bonds due 2021 | Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of debt	[2]	$ 459.3	$ 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]	$ 299.7
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]	$ 497.7
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]	$ 249.6	$ 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]	$ 222.5	$ 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]	$ 182.6	$ 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]	$ 167.5	$ 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]	$ 155.0	$ 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]	$ 144.3	$ 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]	$ 149.7	$ 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]	$ 139.4	$ 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]	$ 92.9	$ 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]	$ 83.7	$ 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]	$ 126.7	$ 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]	$ 107.3	$ 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]	$ 110.1	$ 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]	$ 108.0	$ 101.8

[1] Fair values are based on Level 1 quoted market rates, except for the 4.00% Notes due 2027 and 3.15% Notes Due 2023, which are 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.