Fair Value Measurements (Carrying Value of Debt) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Unamortized debt issuance costs and discounts		$ 13.8	$ 14.2
Carrying Amount | Synthetic bonds due 2021
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[1]	499.2	428.0
Carrying Amount | 3.45% Senior Notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[1]	500.0
Carrying Amount | 5.00% Notes due 2020
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[1]	238.9	209.7
Carrying Amount | 3.40% Notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[1]	179.8	158.0
Carrying Amount | 3.15% Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[1]	155.0	136.1
Carrying Amount | 3.15% Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[1]	149.6	131.4
Carrying Amount | 4.00% Notes due 2027
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[1]	89.9	79.0
Carrying Amount | 4.00% Notes due 2032
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[1]	115.4	101.2
Carrying Amount | 3.75% Notes due 2033
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[1]	116.0	101.8
Fair Value | Synthetic bonds due 2021
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[2]	647.4	661.5
Fair Value | 3.45% Senior Notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[2]	497.7
Fair Value | 5.00% Notes due 2020
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[2]	265.2	239.0
Fair Value | 3.40% Notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[2]	199.4	177.8
Fair Value | 3.15% Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[2]	167.6	153.1
Fair Value | 3.15% Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[2]	161.4	142.8
Fair Value | 4.00% Notes due 2027
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[2]	99.9	89.6
Fair Value | 4.00% Notes due 2032
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[2]	142.9	127.3
Fair Value | 3.75% Notes due 2033
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Senior notes	[2]	$ 126.7	$ 107.8

[1] Carrying amounts are shown net of unamortized debt discounts and premiums and unamortized debt issuance costs of $13.8 million and $14.2 million as of 2017 and 2016, respectively.

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