Stock Compensation Plans - Performance Units/RSU (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Weighted-Average Grant Date Fair Value
Excess tax benefits from stock-based compensation recorded to APIC	$ 0.1	$ 1.2	$ 4.5
Performance units
Stock Compensation Plans
Granted (in shares)	49,730	40,747	41,883
Percentage of target to be awarded, high end of range	200.00%
Common stock earned as a percentage of the performance unit target	67.00%
Market price at grant date of performance units(in USD per share)	$ 69.05
Unvested stock-based awards (other than stock options)
Outstanding at the beginning of the period (in shares)	47,221	41,377	53,506
Shares granted (in shares)	49,730	40,747	41,883
Shares vested (in shares)	0	0	0
Performance Shares vested (in shares)	25,833	31,421	53,506
Shares expired or cancelled (in shares)	(4,927)	(3,482)	(506)
Outstanding at the end of the period (in shares)	66,191	47,221	41,377
Weighted-Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share)	$ 93.21	$ 81.85	$ 73.79
Shares granted (in dollars per share)	69.05	93.21	81.85
Shares vested (in dollars per share)	0	0	0
Performance shares vested (in dollars per share)	93.21	81.85	73.79
Shares expired or cancelled (in dollars per share)	85.67	84.45	81.85
Outstanding at the end of the period (in dollars per share)	$ 75.62	$ 93.21	$ 81.85
RSUs
Stock Compensation Plans
Granted (in shares)	46,556	10,618	10,318
Unvested stock-based awards (other than stock options)
Outstanding at the beginning of the period (in shares)	53,460	88,799	80,719
Shares granted (in shares)	46,556	10,618	10,318
Shares vested (in shares)	(63,595)	(72,190)	(72,451)
Performance Shares vested (in shares)	10,354	33,928	73,838
Shares expired or cancelled (in shares)	(2,113)	(7,695)	(3,625)
Outstanding at the end of the period (in shares)	44,662	53,460	88,799
Weighted-Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share)	$ 67.53	$ 53.33	$ 54.91
Shares granted (in dollars per share)	67.04	82.29	76.84
Shares vested (in dollars per share)	72.91	60.24	55.26
Performance shares vested (in dollars per share)	93.21	88.40	52.11
Shares expired or cancelled (in dollars per share)	69.35	84.45	50.48
Outstanding at the end of the period (in dollars per share)	$ 65.23	$ 67.53	$ 53.33
Units issued in lieu of dividends (in shares)	43	132	226
Aggregate pre-tax intrinsic value of outstanding RSUs	$ 3.1
Aggregate pre-tax intrinsic value of restricted stock and RSUs that vested during the period	$ 4.2	$ 4.4	$ 6.3
RSUs | Non-employee members of the board of directors
Stock Compensation Plans
Granted (in shares)	10,056
Market price at grant date of performance units(in USD per share)	$ 67.04
Vesting period	1 year
Unvested stock-based awards (other than stock options)
Shares granted (in shares)	10,056
RSUs | Employees
Stock Compensation Plans
Granted (in shares)	36,457
Unvested stock-based awards (other than stock options)
Shares granted (in shares)	36,457