Share-Based Compensation, Nonvested Stock (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Forfeited, number of shares	(14,198)	(10,777)	(5,995)
Nonvested Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested stock, number of shares, beginning of period	220,195
Nonvested stock, weighted-average grant date fair value, beginning of period	$ 101.40
Granted, number of shares	85,199
Granted, weighted-average grant date fair value	$ 147.82	$ 114.69	$ 106.52
Vested, number of shares	(97,165)
Vested, weighted-average grant date fair value	$ 104.86
Forfeited, number of shares	(14,198)
Forfeited, weighted-average grant date fair value	$ 110.94
Nonvested stock, number of shares, end of period	194,031	220,195
Nonvested stock, weighted-average grant date fair value, end of period	$ 119.35	$ 101.40
Nonvested stock, aggregate intrinsic value	$ 39.6
Shares vested, fair value	15.3	$ 15.2	$ 10.4
Compensation expense	10.5	10.7	10.8
Income tax benefit recognized in the consolidated statements of income	2.6	$ 2.6	$ 2.6
Unrecognized compensation cost relating to nonvested stock	$ 11.9
Unrecognized compensation cost weighted-average period of recognition	1 year 2 months 12 days
Nonvested Stock [Member] | Minimum [Member] | Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Nonvested Stock [Member] | Maximum [Member] | Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	5 years