Stock-Based Compensation Plans - Restricted Stock (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Additional share-based compensation disclosures
Total unrecognized compensation cost	$ 48.8
Weighted average recognition period for unrecognized compensation cost (in years)	1 year 4 months 6 days
Restricted stock, 2018 grant
Shares
Awards granted (in shares)	474,715
The number of grants made during the period on the basis of service and performance criteria other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).	178,970
Fair value of restricted stock granted (in dollars per share)	$ 84.26
Share of common stock	1
Changes
Granted (in shares)	474,715
Weighted Average Grant Date Fair Value
Granted (in dollars per shares)	$ 84.26
Restricted stock, 2017 grant
Shares
Awards granted (in shares)		446,525
The number of grants made during the period on the basis of service and performance criteria other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).		169,009
Fair value of restricted stock granted (in dollars per share)		$ 79.60
Share of common stock		1
Changes
Granted (in shares)		446,525
Weighted Average Grant Date Fair Value
Granted (in dollars per shares)		$ 79.60
Restricted stock, 2016 grant
Shares
Awards granted (in shares)			512,895
The number of grants made during the period on the basis of service and performance criteria other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).			190,175
Fair value of restricted stock granted (in dollars per share)			$ 69.16
Share of common stock			1
Changes
Granted (in shares)			512,895
Weighted Average Grant Date Fair Value
Granted (in dollars per shares)			$ 69.16
Restricted stock and RSUs
Shares
Awards granted (in shares)	474,715
Fair value of restricted stock granted (in dollars per share)	$ 84.26
Changes
Unvested at the beginning of the period (in shares)	924,575
Granted (in shares)	474,715
Vested (in shares)	(484,858)
Cancelled or forfeited (in shares)	(24,602)
Unvested at the end of the period (in shares)	889,830	924,575
Weighted Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share)	$ 74.09
Granted (in dollars per shares)	84.26
Vested (in dollars per shares)	69.32
Cancelled or forfeited (in dollars per shares)	76.72
Unvested at the end of the period (in dollars per shares)	$ 82.05	$ 74.09
Restricted stock
Shares
Payments to tax authorities on employees' behalf for shares withheld related to share settlements	$ 12.3	$ 9.3	$ 6.4
Performance-based | RSU's
Shares
Performance target period (in years)	3 years
Directors Equity Plan | Stock Awards
Shares
Awards granted (in shares)	13,880	18,120	11,851
Fair value of restricted stock granted (in dollars per share)	$ 93.65	$ 71.73	$ 70.88
Changes
Granted (in shares)	13,880	18,120	11,851
Weighted Average Grant Date Fair Value
Granted (in dollars per shares)	$ 93.65	$ 71.73	$ 70.88