Fair Value Measurements (Carrying Value of Debt) (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net		$ 9.1	$ 11.4
Reported Value Measurement | Synthetic bonds due 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	492.9	490.9
Reported Value Measurement | 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 | 5.00% Notes due 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	224.6	229.0
Reported Value Measurement | 3.40% Notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	168.5	171.8
Reported Value Measurement | 3.15% Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	146.0	148.9
Reported Value Measurement | 3.15% Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	140.4	143.1
Reported Value Measurement | 4.00% Notes due 2027
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	84.2	85.9
Reported Value Measurement | 4.00% Notes due 2032
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	112.3	114.5
Reported Value Measurement | 3.75% Notes due 2033
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[1]	112.3	114.5
Estimate of Fair Value Measurement | Synthetic bonds due 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	513.1	532.4
Estimate of Fair Value Measurement | 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]	499.2	489.7
Estimate of Fair Value Measurement | 5.00% Notes due 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	230.0	244.0
Estimate of Fair Value Measurement | 3.40% Notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	180.6	186.9
Estimate of Fair Value Measurement | 3.15% Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	156.8	161.3
Estimate of Fair Value Measurement | 3.15% Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	150.5	153.3
Estimate of Fair Value Measurement | 4.00% Notes due 2027
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	96.4	95.8
Estimate of Fair Value Measurement | 4.00% Notes due 2032
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	127.8	120.2
Estimate of Fair Value Measurement | 3.75% Notes due 2033
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	[2]	$ 123.8	$ 126.1

[1] Carrying amounts include unamortized debt discounts and premiums and unamortized debt issuance costs of $9.1 million and $11.4 million as of 2019 and 2018, respectively.

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