Stock Compensation Plans - Performance Units/RSU (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted-Average Grant Date Fair Value
Excess tax benefits from stock-based compensation recorded to APIC	$ 4.5	$ 3.1	$ 2.6
Performance units
Stock Compensation Plans
Granted (in shares)	41,883	54,364	45,060
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	101.00%
Market price at grant date of performance units(in USD per share)	$ 82.12
Unvested stock-based awards (other than stock options)
Outstanding at the beginning of the period (in shares)	53,506	43,050	58,270
Shares granted	41,883	54,364	45,060
Shares vested	0	0	(810)
Performance Shares vested	53,506	43,050	58,270
Shares expired or cancelled	(506)	(858)	(1,200)
Outstanding at the end of the period (in shares)	41,377	53,506	43,050
Weighted-Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share)	$ 73.79	$ 78.32	$ 74.79
Shares granted (in dollars per share)	81.85	73.82	78.32
Shares vested (in dollars per share)	0.00	0.00	78.32
Performance shares vested (in dollars per share)	73.79	78.32	74.79
Shares expired or cancelled (in dollars per share)	81.85	75.98	78.32
Outstanding at the end of the period (in dollars per share)	$ 81.85	$ 73.79	$ 78.32
RSUs
Stock Compensation Plans
Granted (in shares)	10,318	10,047	13,415
Unvested stock-based awards (other than stock options)
Outstanding at the beginning of the period (in shares)	80,719	118,838	105,294
Shares granted	10,318	10,047	13,415
Shares vested	(72,451)	(110,749)	(105,564)
Performance Shares vested	73,838.28	(62,874)	107,219
Shares expired or cancelled	(3,625)	(291)	(1,526)
Outstanding at the end of the period (in shares)	88,799	80,719	118,838
Weighted-Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share)	$ 54.91	$ 43.29	$ 31.15
Shares granted (in dollars per share)	76.84	68.25	61.41
Shares vested (in dollars per share)	55.26	42.96	32.12
Performance shares vested (in dollars per share)	52.11	53.63	40.65
Shares expired or cancelled (in dollars per share)	50.48	40.65	51.14
Outstanding at the end of the period (in dollars per share)	$ 53.33	$ 54.91	$ 43.29
Units issued in lieu of dividends (in shares)	226	312	495
Aggregate pre-tax intrinsic value of outstanding RSUs	$ 8.0
Aggregate pre-tax intrinsic value of restricted stock and RSUs that vested during the period	$ 6.3	$ 9.3	$ 6.6
RSUs | Non-employee members of the board of directors
Stock Compensation Plans
Granted (in shares)	9,226
Market price at grant date of performance units(in USD per share)	$ 76.78
Vesting period	1 year
Unvested stock-based awards (other than stock options)
Shares granted	9,226
RSUs | Employees
Stock Compensation Plans
Granted (in shares)	866
Unvested stock-based awards (other than stock options)
Shares granted	866