Fair Value Measurements (Carrying Value of Debt) (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net		$ 12.8	$ 9.1
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]	551.2	492.9
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]	0.0	224.6
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]	184.0	168.5
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]	159.5	146.0
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]	153.4	140.4
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]	92.0	84.2
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]	122.7	112.3
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]	122.7	112.3
Reported Value Measurement | 4.50% Notes due 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value		245.4	0.0
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]	552.0	513.1
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]	513.2	499.2
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]	0.0	230.0
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]	188.8	180.6
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]	163.7	156.8
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]	161.8	150.5
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]	99.7	96.4
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]	136.8	127.8
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]	126.4	123.8
Estimate of Fair Value Measurement | 4.50% Notes due 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value		$ 256.8	$ 0.0

[1] Carrying amounts include unamortized debt discounts and premiums and unamortized debt issuance costs of $12.8 million and $9.1 million as of December 31, 2020, and 2019, respectively.

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