Stock Compensation Plans - Performance Units/RSU (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted-Average Grant Date Fair Value
Excess tax benefits from stock-based compensation recorded to APIC	$ 1.2	$ 4.5	$ 3.1
Performance units
Stock Compensation Plans
Granted (in shares)	40,747	41,883	54,364
Percentage of target to be awarded, low end of range	40.00%
Percentage of target to be awarded, high end of range	200.00%
Common stock earned as a percentage of the performance unit target	40.00%
Market price at grant date of performance units(in USD per share)	$ 93.21
Unvested stock-based awards (other than stock options)
Outstanding at the beginning of the period (in shares)	41,377	53,506	43,050
Shares granted	40,747	41,883	54,364
Shares vested	0	0	0
Performance Shares vested	(31,421)	(53,506)	(43,050)
Shares expired or cancelled	(3,482)	(506)	(858)
Outstanding at the end of the period (in shares)	47,221	41,377	53,506
Weighted-Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share)	$ 81.85	$ 73.79	$ 78.32
Shares granted (in dollars per share)	93.21	81.85	73.82
Shares vested (in dollars per share)	0.00	0.00	0.00
Performance shares vested (in dollars per share)	81.85	73.79	78.32
Shares expired or cancelled (in dollars per share)	84.45	81.85	75.98
Outstanding at the end of the period (in dollars per share)	$ 93.21	$ 81.85	$ 73.79
RSUs
Stock Compensation Plans
Granted (in shares)	10,618	10,318	10,047
Unvested stock-based awards (other than stock options)
Outstanding at the beginning of the period (in shares)	88,799	80,719	118,838
Shares granted	10,618	10,318	10,047
Shares vested	(72,190)	(72,451)	(110,749)
Performance Shares vested	33,928	73,838	62,874
Shares expired or cancelled	(7,695)	(3,625)	(291)
Outstanding at the end of the period (in shares)	53,460	88,799	80,719
Weighted-Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share)	$ 53.33	$ 54.91	$ 43.29
Shares granted (in dollars per share)	82.29	76.84	68.25
Shares vested (in dollars per share)	60.24	55.26	42.96
Performance shares vested (in dollars per share)	88.40	52.11	53.63
Shares expired or cancelled (in dollars per share)	84.45	50.48	40.65
Outstanding at the end of the period (in dollars per share)	$ 67.53	$ 53.33	$ 54.91
Units issued in lieu of dividends (in shares)	132	226	312
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	$ 4.4	$ 6.3	$ 9.3
RSUs | Non-employee members of the board of directors
Stock Compensation Plans
Granted (in shares)	8,456
Market price at grant date of performance units(in USD per share)	$ 82.29
Vesting period	1 year
Unvested stock-based awards (other than stock options)
Shares granted	8,456
RSUs | Employees
Stock Compensation Plans
Granted (in shares)	2,030
Vesting period	3 years
Unvested stock-based awards (other than stock options)
Shares granted	2,030