Equity - Share Repurchases, Dividends, Stock-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions				1 Months Ended	3 Months Ended				6 Months Ended
	Jul. 21, 2020	May 20, 2020	Oct. 23, 2018	Mar. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Share Repurchase Plan
Increase in authorized number of shares to be repurchased			5,000,000.0
Number of shares authorized by the Board of Directors to be repurchased under share repurchase plan					3,100,000				3,100,000
Number of shares authorized by the Board of Directors to be repurchased under share repurchase plan, expressed as a percentage of outstanding shares (as a percent)					5.00%				5.00%
Share repurchases (in shares)					0				3,300,000	600,000
Average costs per share									$ 90.09	$ 84.33
Value of shares repurchased						$ 300.0	$ 50.0		$ 300.0	$ 50.0
Dividends
Common stock quarterly dividend per share (in dollars per share)	$ 0.625				$ 0.625	$ 0.625	$ 0.55	$ 0.55
Common stock dividend quarterly declared and paid per share (in dollars per share)					$ 0.625		$ 0.55		$ 1.25	$ 1.10
Dividends paid					$ 39.9		$ 36.9		$ 80.9	$ 73.9
Directors Equity Plan
Stock-Based Compensation
Extension period of plan		10 years
Common Stock and Additional Paid in Capital
Share Repurchase Plan
Value of shares repurchased						$ 119.3	$ 50.0
Common Stock and Additional Paid-In (in Shares)
Payments made to tax authorities on employees' behalf									5.2	9.6
RSU's
Dividends
Dividend equivalents paid									$ 0.9	$ 2.6
Stock-Based Compensation
Vesting period (in years)									3 years
RSU's | Service-based
Stock-Based Compensation
Vesting period (in years)				3 years
Changes
Granted (in shares)				330,144
Weighted Average Grant Date Fair Value
Granted (in dollars per share)				$ 82.81
RSU's | Performance-based
Stock-Based Compensation
Vesting period (in years)				3 years
Changes
Granted (in shares)				210,403
Weighted Average Grant Date Fair Value
Granted (in dollars per share)				$ 82.81
Unvested RSUs
Stock-Based Compensation
Increase in number of shares available for issuance under the plan (in shares)		1,500,000
Changes
Unvested at the beginning of the year (in shares)						859,005			859,005
Granted (in shares)									540,547
Vested (in shares)									(4,039)
Cancelled or forfeited (in shares)									(13,985)
Unvested at the end of the period (in shares)					1,381,528				1,381,528
Shares reserved for future grants (all plans)					1,992,565				1,992,565
Weighted Average Grant Date Fair Value
Unvested at the beginning of the year (in dollars per share)						$ 86.40			$ 86.40
Granted (in dollars per share)									82.81
Vested (in dollars per share)									85.32
Cancelled or forfeited (in dollars per share)									85.67
Unvested at the end of the period (in dollars per share)					$ 85.01				$ 85.01