Fair Value Measurements (Carrying Value of Debt) (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net		$ 11.4	$ 13.8
Reported Value Measurement [Member] | Synthetic bonds due 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	490.9	502.4
Reported Value Measurement [Member] | 3.45% Senior Notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	500.0	500.0
Reported Value Measurement [Member] | 5.00% Notes due 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	229.0	239.9
Reported Value Measurement [Member] | 3.40% Notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	171.8	179.9
Reported Value Measurement [Member] | 3.15% Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	148.9	155.9
Reported Value Measurement [Member] | 3.15% Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	143.1	149.9
Reported Value Measurement [Member] | 4.00% Notes due 2027
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	85.9	89.9
Reported Value Measurement [Member] | 4.00% Notes due 2032
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	114.5	119.9
Reported Value Measurement [Member] | 3.75% Notes due 2033
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	114.5	119.9
Estimate of Fair Value Measurement [Member] | Synthetic bonds due 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	532.4	647.4
Estimate of Fair Value Measurement [Member] | 3.45% Senior Notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	489.7	497.7
Estimate of Fair Value Measurement [Member] | 5.00% Notes due 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	244.0	265.2
Estimate of Fair Value Measurement [Member] | 3.40% Notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	186.9	199.4
Estimate of Fair Value Measurement [Member] | 3.15% Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	161.3	167.6
Estimate of Fair Value Measurement [Member] | 3.15% Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	153.3	161.4
Estimate of Fair Value Measurement [Member] | 4.00% Notes due 2027
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	95.8	99.9
Estimate of Fair Value Measurement [Member] | 4.00% Notes due 2032
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	120.2	142.9
Estimate of Fair Value Measurement [Member] | 3.75% Notes due 2033
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	$ 126.1	$ 126.7

[1] Carrying amounts include unamortized debt discounts and premiums and unamortized debt issuance costs of $11.4 million and $13.8 million as of 2018 and 2017, respectively.

[2] Fair values are based on Level 2 quoted market rates.