General - Stock Compensation (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) company $ / shares shares	Mar. 31, 2017 USD ($)
Stock options, Non vested restricted stock awards and nonvested restricted stock units
Stock compensation expense | $	$ 7.6	$ 10.1
Benefits of tax deductions in excess of recognized stock compensation expense | $	$ 4.1	$ 0.9
Minimum
Stock options, Non vested restricted stock awards and nonvested restricted stock units
Estimated forfeitures (as a percent)	0.00%	0.00%
Maximum
Stock options, Non vested restricted stock awards and nonvested restricted stock units
Estimated forfeitures (as a percent)	4.00%	4.00%
Stock options
Stock options, Non vested restricted stock awards and nonvested restricted stock units
Grants in period, weighted average grant date fair value (in dollars per share)	$ 25.79
Expected volatility (as a percent)	28.21%
Stock option activity
Outstanding, beginning of period (in shares) | shares	2,458,237
Granted (in shares) | shares	399,706
Forfeited (in shares) | shares	(8,321)
Exercised (in shares) | shares	(299,873)
Outstanding, end of period (in shares) | shares	2,549,749
Vested and expected to vest end of period (in shares) | shares	2,522,274
Exercisable, end of period (in shares) | shares	1,537,926
Weighted average exercise price of stock options
Outstanding, beginning of period (in dollars per share)	$ 61.50
Granted (in dollars per share)	99.46
Forfeited (in dollars per share)	48.01
Exercised (in dollars per share)	57.14
Outstanding, end of period (in dollars per share)	67.96
Vested and expected to vest end of period (in dollars per share)	59.59
Exercisable, end of period (in dollars per share)	$ 67.76
Assumptions used for estimating value of awards granted
Vesting period	3 years
Life of options (Expiration period)	10 years
Restricted stock awards
Nonvested restricted stock awards and units
Outstanding, beginning of period (in shares) | shares	31,102
Outstanding, end of period (in shares) | shares	31,102
Weighted average exercise price of nonvested restricted stock award and units
Outstanding, beginning of period (in dollars per share)	$ 68.00
Outstanding, end of period (in dollars per share)	$ 68.00
Restricted stock awards | Minimum
Assumptions used for estimating value of awards granted
Vesting period	12 months
Restricted stock awards | Maximum
Assumptions used for estimating value of awards granted
Vesting period	24 months
Restricted Stock Units (RSUs)
Nonvested restricted stock awards and units
Outstanding, beginning of period (in shares) | shares	163,289
Awarded (in shares) | shares	109,348
Vested (in shares) | shares	(80,070)
Forfeited (in shares) | shares	(4,349)
Outstanding, end of period (in shares) | shares	188,218
Weighted average exercise price of nonvested restricted stock award and units
Outstanding, beginning of period (in dollars per share)	$ 66.46
Awarded (in dollars per share)	99.45
Vested (in dollars per share)	65.41
Forfeited (in dollars per share)	67.60
Outstanding, end of period (in dollars per share)	$ 86.05
Assumptions used for estimating value of awards granted
Vesting period	3 years
Performance Based Restricted Stock Units (“PSUs”)
Nonvested restricted stock awards and units
Outstanding, beginning of period (in shares) | shares	202,315
Awarded (in shares) | shares	80,502
Vested (in shares) | shares	(33,592)
Forfeited (in shares) | shares	(1,469)
Outstanding, end of period (in shares) | shares	247,756
Estimated fair value	$ 141.61
Weighted average exercise price of nonvested restricted stock award and units
Outstanding, beginning of period (in dollars per share)	84.63
Awarded (in dollars per share)	141.61
Vested (in dollars per share)	85.00
Forfeited (in dollars per share)	85.29
Outstanding, end of period (in dollars per share)	$ 103.09
Assumptions used for estimating value of awards granted
Vesting period	3 years
Expected dividend yield (as a percent)	0.00%
Risk-free interest rate (as a percent)	2.37%
Expected volatility, minimum (as a percent)	20.00%
Expected volatility, maximum (as a percent)	82.00%
Expected volatility, average (as a percent)	35.00%
Number of trading days considered for average share value	30 days
Number of companies in peer group | company	49
Performance Based Restricted Stock Units (“PSUs”) | Minimum
Assumptions used for estimating value of awards granted
Percentage of shares to be settled	0.00%
Performance Based Restricted Stock Units (“PSUs”) | Maximum
Assumptions used for estimating value of awards granted
Percentage of shares to be settled	200.00%