Stock Compensation Plans - Outstanding and Exercisable Stock Options (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Options Vested or Expected to Vest
Number of Options (in shares)	464,958
Weighted-Average Remaining Contractual Life	6 years 6 months 19 days
Weighted-Average Exercise Price (in dollars per share)	$ 55.60
Aggregate Intrinsic Value	$ 16.3
Options Exercisable
Number of Options (in shares)	241,944
Weighted-Average Exercise Price (in dollars per share)	$ 40.59
Aggregate Intrinsic Value	$ 12.1
Closing market price for common stock (in dollars per share)	$ 90.65
Intrinsic value, exercises in the period	$ 11.5	$ 4.7	$ 5.5
$7.41 — $19.25 | Stock Options
Exercise Price
Exercise price, low end of range (in dollars per share)	$ 7.41
Exercise price, high end of range (in dollars per share)	$ 19.25
Options Vested or Expected to Vest
Number of Options (in shares)	52,727
Weighted-Average Remaining Contractual Life	1 year 11 months 6 days
Weighted-Average Exercise Price (in dollars per share)	$ 13.06
Aggregate Intrinsic Value	$ 4.1
Options Exercisable
Number of Options (in shares)	52,727
Weighted-Average Exercise Price (in dollars per share)	$ 13.06
Aggregate Intrinsic Value	$ 4.1
$21.13 — $31.23 | Stock Options
Exercise Price
Exercise price, low end of range (in dollars per share)	$ 21.13
Exercise price, high end of range (in dollars per share)	$ 32.84
Options Vested or Expected to Vest
Number of Options (in shares)	58,490
Weighted-Average Remaining Contractual Life	3 years 10 months 16 days
Weighted-Average Exercise Price (in dollars per share)	$ 27.56
Aggregate Intrinsic Value	$ 3.7
Options Exercisable
Number of Options (in shares)	58,490
Weighted-Average Exercise Price (in dollars per share)	$ 27.56
Aggregate Intrinsic Value	$ 3.7
$42.82 — $51.99 | Stock Options
Exercise Price
Exercise price, low end of range (in dollars per share)	$ 35.92
Exercise price, high end of range (in dollars per share)	$ 42.82
Options Vested or Expected to Vest
Number of Options (in shares)	37,526
Weighted-Average Remaining Contractual Life	6 years
Weighted-Average Exercise Price (in dollars per share)	$ 43.49
Aggregate Intrinsic Value	$ 1.8
Options Exercisable
Number of Options (in shares)	37,526
Weighted-Average Exercise Price (in dollars per share)	$ 43.49
Aggregate Intrinsic Value	$ 1.8
$55.49 — $60.56 | Stock Options
Exercise Price
Exercise price, low end of range (in dollars per share)	$ 50.60
Exercise price, high end of range (in dollars per share)	$ 59.72
Options Vested or Expected to Vest
Number of Options (in shares)	172,626
Weighted-Average Remaining Contractual Life	7 years 6 months
Weighted-Average Exercise Price (in dollars per share)	$ 58.68
Aggregate Intrinsic Value	$ 5.5
Options Exercisable
Number of Options (in shares)	76,141
Weighted-Average Exercise Price (in dollars per share)	$ 58.93
Aggregate Intrinsic Value	$ 2.4
$60.56 | Stock Options
Exercise Price
Exercise price, low end of range (in dollars per share)	$ 60.55
Options Vested or Expected to Vest
Number of Options (in shares)	143,589
Weighted-Average Remaining Contractual Life	8 years 6 months 6 days
Weighted-Average Exercise Price (in dollars per share)	$ 82.11
Aggregate Intrinsic Value	$ 1.2
Options Exercisable
Number of Options (in shares)	17,060
Weighted-Average Exercise Price (in dollars per share)	$ 82.15
Aggregate Intrinsic Value	$ 0.1